Publications

The signal-to-noise ratio (SNR) in magnetic resonance imaging (MRI) governs the quality of signal detection and directly impacts the clarity and reliability of the acquired images. Recent advances in metamaterials have enabled lightweight solutions with selective magnetic responses, offering a route to locally boost SNR in targeted anatomical regions but often with compromised field homogeneity. Here, a wireless metamaterial cage constructed from coaxial cables is engineered for homogeneous SNR enhancement at 3.0 T. With its cylindrical geometry and electromagnetic architecture, the device supports circularly polarized resonance through engineered phase-shifted currents, enabling selective and omnidirectional interaction with the rotating B₁^– field to achieve uniform magnetic field distribution. Integrated with the body coil, the device yields a 32-fold SNR enhancement while maintaining comparable homogeneity to the body coil alone, exhibiting only 12.07% variation within the region of interest (ROI). Benchmarking against a state-of-the-art 16-channel extremity coil further shows that the metacage achieves at least 1.94-fold and 2.24-fold higher SNR in axial and coronal planes, respectively, and exhibits substantially lower SNR variation (12.07% compared to 54.83% for the extremity coil). The results establish the metacage as a compelling platform for next-generation wireless MRI technologies.

Machine learning using transformers has shown great potential in medical imaging, but its real-world applicability remains limited due to the scarcity of annotated data. In this study, we propose a practical framework for the few-shot deployment of pretrained MRI transformers in diverse brain imaging tasks. By utilizing the Masked Autoencoder (MAE) pretraining strategy on a large-scale, multi-cohort brain MRI dataset comprising over 31 million slices, we obtain highly transferable latent representations that generalize well across tasks and datasets. For high-level tasks such as classification, a frozen MAE encoder combined with a lightweight linear head achieves state-of-the-art accuracy in MRI sequence identification with minimal supervision. For low-level tasks such as segmentation, we propose MAE-FUnet, a hybrid architecture that fuses multiscale CNN features with pretrained MAE embeddings. This model consistently outperforms other strong baselines in both skull stripping and multi-class anatomical segmentation under data-limited conditions. With extensive quantitative and qualitative evaluations, our framework demonstrates efficiency, stability, and scalability, suggesting its suitability for low-resource clinical environments and broader neuroimaging applications.

Symmetry-protected bound states in the continuum (BICs) support high-quality factor (Q) resonances. As a result, realizing tunable devices based on these states requires approaches that break the symmetry. Tunable mode leakage in terahertz BIC metasurfaces is demonstrated through structural symmetry breaking of an in-plane mirror symmetry. Specifically, a leaky quasi-BIC mode is created through the introduction of lateral asymmetry of the in-plane resonator geometry and through asymmetric out-of-plane tilting using MEMS cantilever actuation. This provides fine-tuned and reconfigurable control of the radiative leakage. Although out-of-plane deformation can, in principle, induce a BIC-to-quasi-BIC transition, the study focuses on quasi-BIC-to-quasi-BIC modulation in order to clearly demonstrate leakage control and to facilitate modal analysis. Experimental measurements supported with full-wave simulations and coupled-mode theory (CMT) reveal distinct leakage and mode behavior for in-plane and out-of-plane symmetry breaking. Importantly, control experiments using symmetrically tilted cantilevers confirm that radiative leakage to the far-field arises from symmetry breaking rather than deformation alone. The dual symmetry-breaking approach enables control over radiative and intrinsic loss through in-plane and out-of-plane symmetry breaking, providing a robust and scalable route toward reconfigurable high-Q terahertz metasurfaces.

Recent advancements in deep learning have enabled the development of generalizable models that achieve state-of-the-art performance across various imaging tasks. Vision Transformer (ViT)-based architectures, in particular, have demonstrated strong feature extraction capabilities when pre-trained on large-scale datasets. In this work, we introduce the Magnetic Resonance Image Processing Transformer (MR-IPT), a ViT-based image-domain framework designed to enhance the generalizability and robustness of accelerated MRI restoration. Unlike conventional deep learning models that require separate training for different acceleration factors, MR-IPT is pre-trained on a large-scale dataset encompassing multiple undersampling patterns and acceleration settings, enabling a unified framework. By leveraging a shared transformer backbone, MR-IPT effectively learns universal feature representations, allowing it to generalize across diverse restoration tasks. Extensive experiments demonstrate that MR-IPT outperforms both CNN-based and existing transformer-based methods, achieving superior quality across varying acceleration factors and sampling masks. Moreover, MR-IPT exhibits strong robustness, maintaining high performance even under unseen acquisition setups, highlighting its potential as a scalable and efficient solution for accelerated MRI. Our findings suggest that transformer-based general models can significantly advance MRI restoration, offering improved adaptability and stability compared to traditional deep learning approaches.

Noise pollution is a persistent environmental concern with severe implications for human health and resources. Acoustic metamaterials offer the potential for thin silencing devices; however, existing designs often lack practical openness and are thereby limited by their functional bandwidths. This paper introduces a novel approach utilizing a phase gradient ultra-open metamaterial (PGUOM) to address these challenges. The PGUOM, characterized by a phase gradient across three unit cells, efficiently transforms incident waves into spoof surface waves, effectively blocking sound while allowing for a high degree of ventilation. Our design provides adjustable openness, accommodates various boundary conditions, and ensures sustained broadband sound insulation. Theoretical, numerical, and experimental validations demonstrate the efficacy of our concept. This innovative approach represents a significant advancement in ventilated acoustic metamaterials, providing both ventilation and high-performance, broadband sound insulation simultaneously.

Recent advances in MRI reconstruction have demonstrated remarkable success through deep learning-based models. However, most existing methods rely heavily on large-scale, task-specific datasets, making reconstruction in data-limited settings a critical yet underexplored challenge. While regularization by denoising (RED) leverages denoisers as priors for reconstruction, we propose Regularization by Neural Style Transfer (RNST), a novel framework that integrates a neural style transfer (NST) engine with a denoiser to enable magnetic field-transfer reconstruction. RNST generates high-field-quality images from low-field inputs without requiring paired training data, leveraging style priors to address limited-data settings. Our experiment results demonstrate RNST’s ability to reconstruct high-quality images across diverse anatomical planes (axial, coronal, sagittal) and noise levels, achieving superior clarity, contrast, and structural fidelity compared to lower-field references. Crucially, RNST maintains robustness even when style and content images lack exact alignment, broadening its applicability in clinical environments where precise reference matches are unavailable. By combining the strengths of NST and denoising, RNST offers a scalable, data-efficient solution for MRI field-transfer reconstruction, demonstrating significant potential for resource-limited settings.

Battery-free wireless sensing in extreme environments, such as conductive solutions, is crucial for long-term, maintenance-free monitoring, eliminating the limitations of battery power and enhancing durability in hard-to-reach areas. However, in such environments, the efficiency of wireless power transfer via radio frequecny (RF) energy harvesting is heavily compromised by signal attenuation and environmental interference, which degrade antenna quality factors and detune resonance frequencies. These limitations create substantial challenges in wirelessly powering miniaturized sensor nodes for underwater environmental monitoring. To overcome these challenges, electrically-shielded coils with coaxially aligned dual-layer conductors are introduced that confine the electric field within the coil’s inner capacitance. This configuration mitigates electric field interaction with the surrounding medium, making the coils ideal for use as near-field antennas in aquatic applications. Leveraging these electrically-shielded coils, a metamaterial-enhanced reader antenna was developed and a 3-axis sensor antenna for an near-field communication (NFC)-based system. The system demonstrated improved spectral stability, preserving resonance frequency and maintaining a high-quality factor. This advancement enabled the creation of a battery-free wireless sensing platform for real-time environmental monitoring in underwater environments, even in highly conductive saltwater with salinity levels of up to 3.5%.

Magnetic resonance imaging (MRI) relies on high-performance receive coils to achieve optimal signal-to-noise ratio (SNR), but conventional designs are often bulky and complex. Recent advancements in metamaterial technology have led to the development of metamaterial-inspired receive coils that enhance imaging capabilities and offer design flexibility. However, these configurations typically face challenges related to reduced adaptability and increased physical footprint. This study introduces a hybrid receive coil design that integrates an array of capacitively-loaded ring resonators directly onto the same plane as the coil, preserving its 2D layout without increasing its size. Both the coil and metamaterial are individually non-resonant at the targeted Larmor frequency, but their mutual coupling induces a resonance shift, achieving a frequency match and forming a hybrid structure with enhanced SNR. Experimental validation on a 3.0 T MRI platform shows that this design allows for adjustable trade-offs between peak SNR and penetration depth, making it adaptable for various clinical imaging scenarios.

Metamaterials hold great potential to enhance the imaging performance of magnetic resonance imaging (MRI) as auxiliary devices, due to their unique ability to confine and enhance electromagnetic fields. Despite their promise, the current implementation of metamaterials faces obstacles for practical clinical adoption due to several notable limitations, including their bulky and rigid structures, deviations from optimal resonance frequency, and inevitable interference with the radiofrequency (RF) transmission field in MRI. Herein, we address these restrictions by introducing a flexible and smart metamaterial that enhances sensitivity by conforming to patient anatomies while ensuring comfort during MRI procedures. The proposed metamaterial selectively amplifies the magnetic field during the RF reception phase by passively sensing the excitation signal strength, remaining “off” during the RF transmission phase. Additionally, the metamaterial can be readily tuned to achieve a precise frequency match with the MRI system through a controlling circuit. The metamaterial presented here paves the way for the widespread utilization of metamaterials in clinical MRI, thereby translating this promising technology to the MRI bedside.

Along with device miniaturization, severe heat accumulation at unexpected nanoscale hotspots attracts wide attentions and urges efficient thermal management. Heat convection is one of the important heat dissipating paths at nanoscale hotspots but its mechanism is still unclear. Here shows the first experimental investigation of the convective heat transfer coefficient at size-controllable nanoscale hotspots. A specially designed structure of a single-layer graphene supported by gold-nanorod array is proposed, in which the gold nanorods generate hundreds of nanometers heating sources under laser irradiation and the graphene layer works as a temperature probe in Raman thermometry. The determined convective heat transfer coefficient (1928+155 −147 W m⁻² K⁻¹ for the 330 nm hotspot and 1793+157 −159 W m⁻² K⁻¹ for the 240 nm hotspot) is about three orders of magnitude higher than that of nature convection, when the simultaneous interfacial heat conduction and radiation are carefully evaluated. Heat convection, thus, accounts to more than half of the total energy transferred across the graphene/gold nanorods interface. Both the plasmon induced nanoscale hotspots and ballistic convection of air molecules contribute to the enhanced heat convection. This work reveals the importance of heat convection at nanoscale hotspots to the accurate thermal design of miniaturized electronics and further offers a new way to evaluate the convective heat transfer coefficient at nanoscale hotspots.

Acoustic wave modulation plays a pivotal role in various applications, including sound-field reconstruction, wireless communication, and particle manipulation, among others. However, current acoustic metamaterial and metasurface designs typically focus on controlling either reflection or transmission waves, often overlooking the coupling between the amplitude and phase of acoustic waves. To fill this gap, we propose and experimentally validate a design enabling complete control of reflected and transmitted acoustic waves individually across a frequency range of 4 to 8 kHz, allowing arbitrary combinations of amplitude and phase for reflected and transmitted sound in a broadband manner. Additionally, we demonstrate the significance of our approach for sound manipulation by achieving acoustic diffusion, reflection, and focusing, and generating a two-sided three-dimensional hologram at three distinct frequencies. These findings open an alternative avenue for extensively engineering sound waves, promising applications in acoustics and related fields.

Deep learning-based MRI reconstruction models have achieved superior performance these days. Most recently, diffusion models have shown remarkable performance in image generation, in-painting, super-resolution, image editing and more. As a generalized diffusion model, cold diffusion further broadens the scope and considers models built around arbitrary image transformations such as blurring, down-sampling, etc. In this paper, we propose a k-space cold diffusion model that performs image degradation and restoration in k-space without the need for Gaussian noise. We provide comparisons with multiple deep learning-based MRI reconstruction models and perform tests on a well-known large open-source MRI dataset. Our results show that this novel way of performing degradation can generate high-quality reconstruction images for accelerated MRI.

A body area network involving wearable sensors distributed around the human body can continuously monitor physiological signals, finding applications in personal healthcare and athletic evaluation. Existing solutions for near-field body area networks, while facilitating reliable and secure interconnection among battery-free sensors, face challenges including limited spectral stability against external interference. Here we demonstrate a textile metamaterial featuring a coaxially-shielded internal structure designed to mitigate interference from extraneous loadings. The metamaterial can be patterned onto clothing to form a scalable, customizable network, enabling communication between near-field reading devices and battery-free sensing nodes placed within the network. Proof of concept demonstration shows the metamaterial’s robustness against mechanical deformation and exposure to lossy, conductive saline solutions, underscoring its potential applications in wet environments, particularly in athletic activities involving water or significant perspiration, offering insights for the future development of radio frequency components for a robust body area network at a system level.

Diatom exoskeletons, known as frustules, exhibit a unique multilayer structure that has attracted considerable attention across interdisciplinary research fields as a source of biomorphic inspiration. These frustules possess a hierarchical porous structure, ranging from millimeter-scale foramen pores to nanometer-scale cribellum pores. In this study, this natural template for nanopattern design is leveraged to showcase metamaterials that integrates perfect absorption and subwavelength color printing. The cribellum-inspired hierarchical nanopatterns, organized in a hexagonal unit cell with a periodicity of 300 nm, are realized through a single-step electron beam lithography process. By employing numerical models, it is uncovered that an additional induced collective dipole mode is the key mechanism responsible for achieving outstanding performance in absorption, reaching up to 99%. Analysis of the hierarchical organization reveals that variations in nanoparticle diameter and inter-unit-cell distance lead to shifts and broadening of the resonance peaks. It is also demonstrated that the hierarchical nanopatterns are capable of color reproduction with high uniformity and fidelity, serving as hexagonal pixels for high-resolution color printing. These cribellum-inspired metamaterials offer a novel approach to multifunctional metamaterial design, presenting aesthetic potential applications in the development of robotics and wearable electronic devices, such as smart skin or surface coatings integrated with energy harvesting functionalities.

Anatomy-specific radio frequency receive coil arrays routinely adopted in magnetic resonance imaging (MRI) for signal acquisition are commonly burdened by their bulky, fixed, and rigid configurations, which may impose patient discomfort, bothersome positioning, and suboptimal sensitivity in certain situations. Herein, leveraging coaxial cables’ inherent flexibility and electric field confining property, we present wireless, ultralightweight, coaxially shielded, passive detuning MRI coils achieving a signal-to-noise ratio comparable to or surpassing that of commercially available cutting-edge receive coil arrays with the potential for improved patient comfort, ease of implementation, and substantially reduced costs. The proposed coils demonstrate versatility by functioning both independently in form-fitting configurations, closely adapting to relatively small anatomical sites, and collectively by inductively coupling together as metamaterials, allowing for extension of the field of view of their coverage to encompass larger anatomical regions without compromising coil sensitivity. The wireless, coaxially shielded MRI coils reported herein pave the way toward next-generation MRI coils.

Bound states in the continuum (BIC) are a non-radiative state embedded in a continuous spectrum of radiating waves. BICs have emerged as a promising platform for opto-electronic phenomena that are dependent on high quality factors. However, the quality factor of metallic metamaterial-based BICs is limited due to ohmic loss, even at terahertz frequencies. As an alternative, we investigate active all-silicon BIC terahertz metamaterials. Quasi-BIC states can be realized either through structurally symmetry breaking or by changing the incident angle of the terahertz waves, as verified with coupled mode theory (CMT). Samples fabricated using micromachining techniques were characterized using terahertz time domain spectroscopy revealing good agreement with simulations. Moreover, we investigated optical tuning of the quasi-BIC response using low-fluence (< 25 µJ/cm²) excitation with 1.5 eV pulses. The dynamic response is consistent with full-wave electromagnetic simulations and indicate that all-silicon metamaterials are a viable active BIC platform with potential applications including terahertz sensing and terahertz nonlinear lightwave phenomena.

Recent advancements in metamaterials have yielded the possibility of a wireless solution to improve signal-to-noise ratio (SNR) in magnetic resonance imaging (MRI). Unlike traditional closely packed local coil arrays with rigid designs and numerous components, these lightweight, cost-effective metamaterials eliminate the need for radio frequency cabling, baluns, adapters, and interfaces. However, their clinical adoption is limited by their low sensitivity, bulky physical footprint, and limited, specific use cases. Herein, a wearable metamaterial developed using commercially available coaxial cable, designed for a 3.0 T MRI system is introduced. This metamaterial inherits the coaxially-shielded structure of its constituent cable, confining the electric field within and mitigating coupling to its surroundings. This ensures safer clinical adoption, lower signal loss, and resistance to frequency shifts. Weighing only 50 g, the metamaterial maximizes its sensitivity by conforming to the anatomical region of interest. MRI images acquired using this metamaterial with various pulse sequences achieve an SNR comparable or even surpass that of a state-of-the-art 16-channel knee coil. This work introduces a novel paradigm for constructing metamaterials in the MRI environment, paving the way for the development of next-generation wireless MRI technology.

Metamaterials hold significant promise for enhancing the imaging capabilities of magnetic resonance imaging (MRI) machines as an additive technology, due to their unique ability to enhance local magnetic fields. However, despite their potential, the metamaterials reported in the context of MRI applications have often been impractical. This impracticality arises from their predominantly flat configurations and their susceptibility to shifts in resonance frequencies, preventing them from realizing their optimal performance. Here, a computational method for designing wearable and tunable metamaterials via freeform auxetics is introduced. The proposed computational-design tools yield an approach to solving the complex circle packing problems in an interactive and efficient manner, thus facilitating the development of deployable metamaterials configured in freeform shapes. With such tools, the developed metamaterials may readily conform to a patient’s knee, ankle, head, or any part of the body in need of imaging, and while ensuring an optimal resonance frequency, thereby paving the way for the widespread adoption of metamaterials in clinical MRI applications.

Acoustic metamaterials introduce unprecedented ways by which to modulate acoustic waves in amplitude, phase, or both. Reconfigurable acoustic metamaterials yield advantages when compared to conventional metamaterials due to their flexible geometry and feasibility in realizing versatile functions. Herein, a reconfigurable acoustic metamaterial based on the angle-variant unit cells is proposed and demonstrated. Two orientation states of the V-shaped unit cell are selected to maximize the phase-shift-modulation effect. Furthermore, a metamaterial consisting of 18 components is fabricated, each of which is composed of 18 V-shaped unit cells in different orientation states. The components of the metamaterial may be readily reconfigured to deliver various phase-shift profiles in order to achieve on-demand acoustic functions. In addition, this rotation mechanism can be predictably improved by integrating motors for dynamic real-time reconfiguration. In this study, a series of acoustic functions, including acoustic focusing, splitting, and diffusion, are numerically and experimentally demonstrated. Additionally, an acoustic blocker design is also proposed. The results we present herein demonstrate the promise of our design in wave control. This work extends the realm of reconfigurable acoustic metamaterials and provides an alternative path for multifunctional acoustic wave modulation.

The application of compressed sensing (CS)-enabled data reconstruction for accelerating magnetic resonance imaging (MRI) remains a challenging problem. This is due to the fact that the information lost in k-space from the acceleration mask makes it difficult to reconstruct an image similar to the quality of a fully sampled image. Multiple deep learning-based structures have been proposed for MRI reconstruction using CS, in both the k-space and image domains, and using unrolled optimization methods. However, the drawback of these structures is that they are not fully utilizing the information from both domains (k-space and image). Herein, we propose a deep learning-based attention hybrid variational network that performs learning in both the k-space and image domains. We evaluate our method on a well-known open-source MRI dataset (652 brain cases and 1172 knee cases) and a clinical MRI dataset of 243 patients diagnosed with strokes from our institution to demonstrate the performance of our network. Our model achieves an overall peak signal-to-noise ratio/structural similarity of 40.92 ± 0.29/0.9577 ± 0.0025 (fourfold) and 37.03 ± 0.25/0.9365 ± 0.0029 (eightfold) for the brain dataset, 31.09 ± 0.25/0.6901 ± 0.0094 (fourfold) and 29.49 ± 0.22/0.6197 ± 0.0106 (eightfold) for the knee dataset, and 36.32 ± 0.16/0.9199 ± 0.0029 (20-fold) and 33.70 ± 0.15/0.8882 ± 0.0035 (30-fold) for the stroke dataset. In addition to quantitative evaluation, we undertook a blinded comparison of image quality across networks performed by a subspecialty trained radiologist. Overall, we demonstrate that our network achieves a superior performance among others under multiple reconstruction tasks.

Magnetic Resonance Imaging (MRI) acceleration techniques using k-space sub-sampling (KS) can greatly improve the efficiency of MRI-based stroke diagnosis. Although Deep Neural Networks (DNN) have shown great potential on stroke lesion recognition tasks when the MR images are reconstructed from the full k-space, they are vulnerable to the lower quality MR images generated by KS. In this paper, we propose a Distributionally Robust Learning (DRL) approach to improve the performance of stroke recognition DNN models when the MR images are reconstructed from the sub-sampled k-space. For Convolutional Neural Network (CNN) and Vision Transformer (ViT)-based models, our methods improve the stroke classification AUROC and AUPRC by up to 11.91% and 9.32% on the KS-perturbed brain MR images, respectively, compared against Empirical Risk Minimization (ERM) and other baseline defensive methods. We further show that DRL models can successfully recognize the stroke cases from highly perturbed MR images where clinicians may fail, which provides a solution for improved diagnosis in an accelerated MRI setting.

Signal-to-noise ratio (SNR) is one of the most common metrics in assessing the image quality of magnetic resonance imaging (MRI). Among a host of technological developments, various wireless devices, including metamaterials and volumetric wireless resonators have been reported to enhance SNR by redistributing the radio frequency magnetic field in the near field region. While theoretically feasible, their widespread clinical adoption has been limited by their field inhomogeneity, limited spatial coverage and challenges in their applications to higher field (≥3.0T) MRI systems. In this study, a Helmholtz coil-inspired volumetric wireless resonator (HVWR) featuring a uniform magnetic field enhancement within the resonator volume is reported. The HVWR is free from cables, adapters and interface boxes, allowing for ease of fabrication and straightforward installation. The resonator allows for resonance frequency tunability and adaptivity, enabling for passive detuning during the MRI transmission phase. Experimental validation using a 3.0T MRI system demonstrate a substantial SNR boost (5× or higher) being achieved in a region covering the average size of the human knee. This study offers an efficient and practical wireless solution for improved MRI image quality that may be applicable across a range of imaging applications.

A central goal of modern magnetic resonance imaging (MRI) is to reduce the time required to produce high-quality images. Efforts have included hardware and software innovations such as parallel imaging, compressed sensing, and deep learning-based reconstruction. Here, we propose and demonstrate a Bayesian method to build statistical libraries of magnetic resonance (MR) images in k-space and use these libraries to identify optimal subsampling paths and reconstruction processes. Specifically, we compute a multivariate normal distribution based upon Gaussian processes using a publicly available library of T1-weighted images of healthy brains. We combine this library with physics-informed envelope functions to only retain meaningful correlations in k-space. This covariance function is then used to select a series of ring-shaped subsampling paths using Bayesian optimization such that they optimally explore space while remaining practically realizable in commercial MRI systems. Combining optimized subsampling paths found for a range of images, we compute a generalized sampling path that, when used for novel images, produces superlative structural similarity and error in comparison to previously reported reconstruction processes (i.e. 96.3% structural similarity and < 0.003 normalized mean squared error from sampling only 12.5% of the k-space data). Finally, we use this reconstruction process on pathological data without retraining to show that reconstructed images are clinically useful for stroke identification. Since the model trained on images of healthy brains could be directly used for predictions in pathological brains without retraining, it shows the inherent transferability of this approach and opens doors to its widespread use.

Bound states in the continuum (BIC) is an exotic concept describing systems without radiative loss. BICs are widely investigated in optics due to numerous potential applications including lasing, sensing, and filtering, among others. This study introduces a structurally tunable BIC terahertz metamaterial fabricated using micromachining and experimentally characterized using terahertz time domain spectroscopy. Control of the bending angle of the metamaterial by thermal actuation modifies the capacitance enabling tuning from a quasi-BIC state with a quality factor of 26 to the BIC state. The dynamic response from the quasi-BIC state to the BIC state is achieved by blueshifting the resonant frequency of the LC mode while maintaining a constant resonant frequency for the dipole mode. Additional insight into the tunable electromagnetic response is obtained using temporal coupled mode theory (CMT). The results reveal the effectiveness of bi-layer cantilever-based structures to realize tunable BIC metamaterials with potential applications for nonlinear optics and light-matter control at terahertz frequencies.

We propose a composite acoustic metamaterial consisting of Mie resonators and a Helmholtz resonator array. Such a design achieves a broadband acoustic attenuation in the low-frequency regime. This wideband soundproofing effect may be explained using the transfer-matrix method and the lumped-element model. Transmission loss and transmittance are robust and tested both numerically as well as experimentally. Through the composite design, using a deep-subwavelength structure, we successfully achieve a broadband low-frequency acoustic attenuation that blocks over 90% of incident acoustic energy within a frequency range of 1250 Hz. Our work offers a design paradigm by which to realize extraordinary airborne acoustic silencing in low-frequency regimes.

High-throughput, rapid and non-invasive readouts of tissue health in microfluidic kidney co-culture models would expand their capabilities for pre-clinical assessment of drug-induced nephrotoxicity. Here, we demonstrate a technique for monitoring steady state oxygen levels in PREDICT96-O₂, a high-throughput organ-on-chip platform with integrated optical-based oxygen sensors, for evaluation of drug-induced nephrotoxicity in a human microfluidic co-culture model of the kidney proximal tubule (PT). Oxygen consumption measurements in PREDICT96-O₂ detected dose and time-dependent injury responses of human PT cells to cisplatin, a drug with known toxic effects in the PT. The injury concentration threshold of cisplatin decreased exponentially from 19.8 µM after 1 day to 2.3 µM following a clinically relevant exposure duration of 5 days. Additionally, oxygen consumption measurements resulted in a more robust and expected dose-dependent injury response over multiple days of cisplatin exposure compared to colorimetric-based cytotoxicity readouts. The results of this study demonstrate the utility of steady state oxygen measurements as a rapid, non-invasive, and kinetic readout of drug-induced injury in high-throughput microfluidic kidney co-culture models.

Ventilated acoustic insulation currently represents one of the most promising research directions in applied acoustics. With the ongoing development and application of acoustic metamaterials, tremendous progress has been made in this space. In this work, we propose a ventilated acoustic insulator based on a labyrinthine metamaterial, a design that consists of a peripheral, circumferential labyrinthine region and a central ventilated, open region. Herein, we demonstrate the potential for this design to yield high-performance wide-band acoustic insulation, a performance metric currently lacking in this class of acoustic silencers, in combination with ventilation. The silencing effect of the labyrinthine acoustic insulator is theoretically and experimentally verified in the frequency range from 1025 to 2000 Hz. Our reported design establishes the foundation for the development of increasingly broadband ventilated acoustic insulators.

One route to create tunable metamaterials is through integration with “on-demand” dynamic quantum materials, such as vanadium dioxide (VO₂). This enables modalities to create high-performance devices for historically challenging applications. Indeed, dynamic materials have often been integrated with metamaterials to imbue artificial structures with some degree of tunability. Conversely, metamaterials can be used to enhance and extend the natural tuning range of dynamic materials. Utilizing a complementary split-ring resonator array deposited on a VO₂ film, we demonstrate enhanced terahertz transmission modulation upon traversing the insulator-to-metal transition (IMT) at approximately 340 K. Our complementary metamaterial increases the modulation amplitude of the original VO₂ film from 0.42 to 0.68 at 0.47 THz upon crossing the IMT, corresponding to an enhancement of 62%. Moreover, temperature-dependent transmission measurements reveal a significant redshift of the resonant frequency in a narrow temperature range where phase coexistence is known to occur. Neither Maxwell-Garnett nor Bruggeman effective medium theory adequately describes the observed frequency shift and amplitude decrease. However, a Drude model incorporating a significant increase of the effective permittivity does describe the experimentally observed redshift. Our results highlight that symbiotic integration of metamaterial arrays with quantum materials provides a powerful approach to engineer emergent functionality.

Measurement of cell metabolism in moderate-throughput to high-throughput organ-on-chip (OOC) systems would expand the range of data collected for studying drug effects or disease in physiologically relevant tissue models. However, current measurement approaches rely on fluorescent imaging or colorimetric assays that are focused on endpoints, require labels or added substrates, and lack real-time data. Here, we integrated optical-based oxygen sensors in a high-throughput OOC platform and developed an approach for monitoring cell metabolic activity in an array of membrane bilayer devices. Each membrane bilayer device supported a culture of human renal proximal tubule epithelial cells on a porous membrane suspended between two microchannels and exposed to controlled, unidirectional perfusion and physiologically relevant shear stress for several days. For the first time, we measured changes in oxygen in a membrane bilayer format and used a finite element analysis model to estimate cell oxygen consumption rates (OCRs), allowing comparison with OCRs from other cell culture systems. Finally, we demonstrated label-free detection of metabolic shifts in human renal proximal tubule cells following exposure to FCCP, a drug known for increasing cell oxygen consumption, as well as oligomycin and antimycin A, drugs known for decreasing cell oxygen consumption. The capability to measure cell OCRs and detect metabolic shifts in an array of membrane bilayer devices contained within an industry standard microtiter plate format will be valuable for analyzing flow-responsive and physiologically complex tissues during drug development and disease research.

Background: Extremely preterm (EP) birth is associated with higher risks of perinatal white matter (WM) injury, potentially causing abnormal neurologic and neurocognitive outcomes. MRI biomarkers distinguishing individuals with and without neurologic disorder guide research on EP birth antecedents, clinical correlates, and prognoses. Purpose: To compare multiparametric quantitative MRI (qMRI) parameters of EP-born adolescents with autism spectrum disorder, cerebral palsy, epilepsy, or cognitive impairment (ie, atypically developing) with those without (ie, neurotypically developing), characterizing sex-stratified brain development. Materials and Methods: This prospective multicenter study included individuals aged 14–16 years born EP (Extremely Low Gestational Age Newborns–Environmental Influences on Child Health Outcomes Study, or ELGAN-ECHO). Participants underwent 3.0-T MRI evaluation from 2017 to 2019. qMRI outcomes were compared for atypically versus neurotypically developing adolescents and for girls versus boys. Sex-stratified multiple regression models were used to examine associations between spatial entropy density (SE_d) and T1, T2, and cerebrospinal fluid (CSF)–normalized proton density (nPD), and between CSF volume and T2. Interaction terms modeled differences in slopes between atypically versus neurotypically developing adolescents. Results: A total of 368 adolescents were classified as 116 atypically (66 boys) and 252 neurotypically developing (125 boys) participants. Atypically versus neurotypically developing girls had lower nPD (mean, 557 10 × percent unit [pu] ± 46 [SD] vs 573 10 × pu ± 43; P = .04), while atypically versus neurotypically developing boys had longer T1 (814 msec ± 57 vs 789 msec ± 82; P = .01). Atypically developing girls versus boys had lower nPD and shorter T2 (eg, in WM, 557 10 × pu ± 46 vs 580 10 × pu ± 39 for nPD [P = .006] and 86 msec ± 3 vs 88 msec ± 4 for T2 [P = .003]). Atypically versus neurotypically developing boys had a more moderate negative association between T1 and SEd (slope, –32.0 msec per kB/cm³ [95% CI: –49.8, –14.2] vs –62.3 msec per kB/cm³ [95% CI: –79.7, –45.0]; P = .03). Conclusion: Atypically developing participants showed sexual dimorphisms in the cerebrospinal fluid–normalized proton density (nPD) and T2 of both white matter (WM) and gray matter. Atypically versus neurotypically developing girls had lower WM nPD, while atypically versus neurotypically developing boys had longer WM T1 and more moderate T1 associations with microstructural organization in WM.

Among approaches aiming toward functional nervous system restoration, those implementing microfabrication techniques allow the manufacture of platforms with distinct geometry where neurons can develop and be guided to form patterned connections in vitro. The interplay between neuronal development and the microenvironment, shaped by the physical limitations, remains largely unknown. Therefore, it is crucial to have an efficient way to quantify neuronal morphological changes induced by physical or contact guidance of the microenvironment. In this study, we first devise and assess a method to prepare anisotropic, gradient poly(dimethylsiloxane) micro-ridge/groove arrays featuring variable local pattern width. We then demonstrate the ability of this single substrate to simultaneously profile the morphologcial and synaptic connectivity changes of primary cultured hippocampal neurons reacting to variable physical conditions, throughout neurodevelopment, in vitro. The gradient microtopography enhanced adhesion within microgrooves, increasing soma density with decreasing pattern width. Decreasing pattern width also reduced dendritic arborization and increased preferential axon growth. Finally, decreasing pattern geometry inhibited presynaptic puncta architecture. Collectively, a method to examine structural development and connectivity in response to physical stimuli is established, and potentially provides insight into microfabricated geometries which promote neural regeneration and repair.

Metasurface absorbers are of particular interest in numerous photonic applications including detectors, photovoltaic cells, and emissivity coatings. We introduce a thin membrane silicon metasurface absorber with periodic elliptical holes that, as demonstrated theoretically and experimentally, achieves very high absorption (≥90%) over a ~500 GHz bandwidth at normal incidence. Based on the analysis of the effective medium theory, the broadband absorption is attributed to proximal electric and magnetic dipole resonances. The absorption amplitude can also be tuned by ~20% with above-gap photoexcitation. Due to the unit cell geometry, the carrier density on the top surface and sidewalls of the membrane must be taken into account. Our dynamic membrane silicon metasurface absorber is notably thin and CMOS-compatible, providing a promising platform to realize compact terahertz devices including detectors, modulators, and switches.

Radiofrequency identification (RFID), particularly passive RFID, is extensively employed in industrial applications to track and trace products, assets, and material flows. The ongoing trend toward increasingly miniaturized RFID sensor tags is likely to continue as technology advances, although miniaturization presents a challenge with regard to the communication coverage area. Recently, efforts in applying metamaterials in RFID technology to increase power transfer efficiency through their unique capacity for electromagnetic wave manipulation have been reported. In particular, metamaterials are being increasingly applied in far-field RFID system applications. Here, we report the development of a magnetic metamaterial and local field enhancement package enabling a marked boost in near-field magnetic strength, ultimately yielding a dramatic increase in the power transfer efficiency between reader and tag antennas. The application of the proposed magnetic metamaterial and local field enhancement package to near-field RFID technology, by offering high power transfer efficiency and a larger communication coverage area, yields new opportunities in the rapidly emerging Internet of Things (IoT) era.

Auxetics refers to structures or materials with a negative Poisson’s ratio, thereby capable of exhibiting counterintuitive behaviors. Herein, auxetic structures are exploited to design mechanically tunable metamaterials in both planar and hemispherical configurations operating at megahertz (MHz) frequencies, optimized for their application to magnetic resonance imaging (MRI). Specially, the reported tunable metamaterials are composed of arrays of interjointed unit cells featuring metallic helices, enabling auxetic patterns with a negative Poisson’s ratio. The deployable deformation of the metamaterials yields an added degree of freedom with respect to frequency tunability through the resultant modification of the electromagnetic interactions between unit cells. The metamaterials are fabricated using 3D printing technology and an ≈20 MHz frequency shift of the resonance mode is enabled during deformation. Experimental validation is performed in a clinical (3.0 T) MRI system, demonstrating that the metamaterials enable a marked boost in radiofrequency field strength under resonance-matched conditions, ultimately yielding a dramatic increase in the signal-to-noise ratio (≈4.5×) of MRI. The tunable metamaterials presented herein offer a novel pathway toward the practical utilization of metamaterials in MRI, as well as a range of other emerging applications.

During the past decade, metasurfaces have shown great potential to complement standard optics, providing novel pathways to control the phase, amplitude, and polarization of electromagnetic waves utilizing arrays of subwavelength resonators. We present dynamic surface wave (SW) switching at terahertz frequencies utilizing a mechanically reconfigurable metasurface. Our metasurface is based on a microelectromechanical system (MEMS) consisting of an array of micro-cantilever structures, enabling dynamic tuning between a plane wave (PW) and a SW for normal incidence terahertz radiation. This is realized using line-by-line voltage control of the cantilever displacements to achieve full-span 2π phase control. Full-wave electromagnetic simulations and terahertz time-domain spectroscopy agree with coupled mode theory, which was employed to design the metasurface device. A conversion efficiency of nearly 60% has been achieved upon switching between the PW and SW configurations. Moreover, a nearly 100 GHz working bandwidth is demonstrated. The MEMS-based control modality we demonstrate can be used for numerous applications, including but not limited to terahertz multifunctional metasurface devices for spatial light modulation, dynamic beam steering, focusing, and beam combining, which are crucial for future “beyond 5G” communication systems.

In this study, we used two-photon polymerization 3D printing technology to successfully print the first true pore-scale rock proxy of Berea sandstone with a submicrometer resolution. Scanning electron microscope (SEM) and computed tomography (CT) images of the 3D-printed sample were compared with the digital file used for printing to verify the rock’s internal structures. Petrophysical properties were estimated with a digital rock physics (DRP) model based on the 3D-printed sample’s initial pore network. The results show that our 3D-printing workflow was able to reproduce true-scale 3D porous media such as Berea sandstone with a submicrometer resolution. With a variety of materials and geometric scaling options, 3D printing of nearly identical rock proxies provides a method to conduct repeatable laboratory experiments without destroying natural rock samples. Rock proxy experiments can potentially validate numerical simulations and complement existing laboratory measurements.

We investigate transient nanotextured heterogeneity in vanadium dioxide (VO₂) thin films during a light-induced insulator-to-metal transition (IMT). Time-resolved scanning near-field optical microscopy (Tr-SNOM) is used to study VO₂ across a wide parameter space of infrared frequencies, picosecond time scales, and elevated steady-state temperatures with nanoscale spatial resolution. Room temperature, steady-state, phonon enhanced nano-optical contrast reveals preexisting “hidden” disorder. The observed contrast is associated with inequivalent twin domain structures. Upon thermal or optical initiation of the IMT, coexisting metallic and insulating regions are observed. Correlations between the transient and steady-state nano-optical textures reveal that heterogeneous nucleation is partially anchored to twin domain interfaces and grain boundaries. Ultrafast nanoscopic dynamics enable quantification of the growth rate and bound the nucleation rate. Finally, we deterministically anchor photoinduced nucleation to predefined nanoscopic regions by locally enhancing the electric field of pump radiation using nanoantennas and monitor the on-demand emergent metallicity in space and time.

Compared with the traditional electric and magnetic multipoles, the existence of a dynamic toroidal moment has received increasing interest in recent years. This is due to its novel electromagnetic response, including dynamic non-radiating charge-current configurations and non-reciprocal interactions. Reconfigurable terahertz metamaterials where artificial toroidal metamolecules and traditional microelectromechanical systems bi-material cantilever structures are integrated within the same unit cell are presented. Through modification of the bending angle by thermal actuation, the toroidal dipole intensity increases by five orders of magnitude in the out-of-plane direction with an overall increase in the toroidal intensity of nearly an order of magnitude. Terahertz time-domain spectroscopy is used to determine the evolution of the transmission as a function of the bending angle. This enables numerical confirmation of the toroidal response using multipole decomposition with additional confirmation provided by phase analysis. The results demonstrate the use of bi-material cantilevers to realize a tunable toroidal moment with potential applications in sensing and next-generation communication technologies.

The resonant modes and their coupling effect of metamaterials is a rapidly growing research topic with potential for applications in optoelectronics. In this work, a strong absorption coupling between the spoof surface plasmon polariton (SSPP) modes and the intrinsic modes has been investigated in a novel terahertz metamaterial. The anticrossing effects of the absorption-mode coupling show a splitting frequency difference that can be described based on a Rabi-splitting-like model. The theoretical and experimental results demonstrate that the splitting phenomenon is induced by an energy exchange between the intrinsic mode and the SSPP mode. This coupling process is mediated by the SSPP mode and can be controlled efficiently by directionally varying the lattice constant of the metamaterial. Furthermore, this absorption process involving hybrid modes can lead to resonances with ultra-high quality factors, which will provide a strong platform for the applications in the field of sensing and detection.

Due to the high-porosity structure, the low thermal transport property of graphene foam (GF) is expected. However, the interconnected skeleton can still act as excellent thermal conductor branches if phonon scattering is not severely affected in the structure of graphene flakes. Such a property has not been validated experimentally due to the difficulty in sample manipulation and the fragility of the structure. In this work, we report the characterization results of thermal properties of the free-standing skeleton in GF. Three individual skeleton samples from one GF piece are prepared under the same condition. The thermal diffusivity of GF skeletons is characterized in the range of 3.26–3.48 × 10⁻⁴ m²/s, and the thermal conductivity is determined to be 520–555 W/(m K), which is two orders of magnitude larger than the value of bulk GF. These high thermal conductivity values originate from the intrinsic thermal property of graphene, while the contact interfaces, wrinkled structures, and defects induced in the synthesis process do not affect the phonon transport property significantly, which proves that the three-dimensional hierarchical graphene structure can still be implemented in energy-intensive applications.

We present a polarization-insensitive air-spaced triple-band metamaterial perfect absorber (MMPA), consisting of a metamaterial layer and metallic ground plane operating at terahertz frequencies. Three near-unity absorption peaks can be individually determined by the geometry of the ring resonators within one unit cell, since the inter-unit-cell coupling is negligible. However, for sufficiently small interlayer spacing (<~20 µ⁢m), coupling between the metamaterial layer and the ground plane is non-negligible. Therefore, near-field interactions must be taken into account for a full understanding of the electromagnetic response (EMR). Interference theory is often used to model the EMR of MMPAs analytically, in which interlayer coupling between the metamaterial and ground plane is usually neglected, resulting in a predicted blueshift of the absorption peaks in comparison to experiment. To account for near-field coupling, we incorporate correction terms into the analytical interference model by taking into account the effective interlayer capacitance and inductance. This results in good agreement between interference theory and experiment (and full-wave numerical simulations). Our findings demonstrate that interlayer coupling is an important design parameter for ultrathin MMPAs.

The concept of “bound states in the continuum” (BIC) describes an idealized physical system exhibiting zero radiative loss composed, for example, of an infinitely extended array of resonators. In principle, vanishing of radiative losses enables an infinitely high-quality factor and corresponding infinite lifetime of the resonance. As such, BIC inspired metasurfaces and photonic designs aim to achieve superior performance in various applications including sensing and lasing. We describe an analytical model based on temporal coupled mode theory to realize an “accidental” (i.e., parameter-tuned) Friedrich–Wintgen BIC. Further, we experimentally verify this model with measurements of quasi-BICs in a metallic terahertz metasurface (MS) and the corresponding complementary metasurface (CMS) using terahertz time domain spectroscopy. For the MS and CMS structures, quality factors of ~20 are achieved, limited by non-radiative intrinsic loss in the materials. Our results reveal that Babinet’s principle qualitatively holds for the MS and CMS quasi-BIC structures. In addition, ultra-high electric and magnetic field enhancement MS and CMS structures, respectively, are presented.

Detecting low energy photons, such as photons in the long-wave infrared range, is a technically challenging proposition using naturally occurring materials. In order to address this challenge, we herein demonstrate a micro-bolometer featuring an integrated metamaterial absorber (MA), which takes advantage of the resonant absorption and frequency selective properties of the MA. Importantly, our micro-bolometer exhibits polarization insensitivity and high absorption due to a novel metal-insulator-metal (MIM) absorber design, operating at 8-12 µm wavelength. The metamaterial structures we report herein feature an interconnected design, optimized towards their application to micro-bolometer-based, long-wave infrared detection. The micro-bolometers were fabricated using a combination of conventional photolithography and electron beam lithography (EBL), the latter owing to the small feature sizes within the design. The absorption response was designed using the coupled mode theory (CMT) and the finite integration technique, with the fabricated devices characterized using Fourier-transform infrared spectroscopy (FTIR). The metamaterial-based micro-bolometer exhibits a responsivity of approximately 198 V/W over the 8-12 µm wavelength regime, detectivity of ~ 0.6 × 109 Jones, thermal response time of ~ 3.3 ms, and a noise equivalent temperature difference (NETD) of ~33 mK under 1mA biasing current at room-temperature and atmosphere pressure. The ultimate detectivity and NETD are limited by Johnson noise and heat loss with thermal convection through air; however, further optimization could be achieved by reducing the thermal conductivity via vacuum packaging. Under vacuum conditions, the detectivity may be increased in excess of two-fold, to ~ 1.5 × 109 Jones. Finally, an infrared image of a soldering iron was generated using a single-pixel imaging process, serving as proof-of-concept of this detection platform. The results presented in this work pave the road towards high-efficiency and frequency-selective detection in the long-wave infrared range through the integration of infrared MAs with micro-bolometers.

Breaking Lorentz reciprocity is fundamental to an array of functional radiofrequency (RF) and optical devices, such as isolators and circulators. The application of external excitation, such as magnetic fields and spatial–temporal modulation, has been employed to achieve nonreciprocal responses. Alternatively, nonlinear effects may also be employed to break reciprocity in a completely passive fashion. Herein, a coupled system comprised of linear and nonlinear meta-atoms that achieves nonreciprocity based on the coupling and frequency detuning of its constituent meta-atoms is presented. An analytical model is developed based on the coupled mode theory (CMT) in order to design and optimize the nonreciprocal meta-atoms in this coupled system. Experimental demonstration of an RF isolator is performed, and the contrast between forward and backward propagation approximates 20 dB. Importantly, the use of the CMT model developed herein enables a generalizable capacity to predict the limitations of nonlinearity-based nonreciprocity, thereby facilitating the development of novel approaches to breaking Lorentz reciprocity. The CMT model and implementation scheme presented in this work may be deployed in a wide range of applications, including integrated photonic circuits, optical metamaterials, and metasurfaces, among others.

Devices designed to dynamically control the transmission, reflection, and absorption of terahertz (THz) radiation are essential for the development of a broad range of THz technologies. A viable approach utilizes materials which undergo an insulator-to-metal transition (IMT), switching from transmissive to reflective upon becoming metallic. However, for many applications, it is undesirable to have spurious reflections that can scatter incident light and induce noise to the system. We present an electrically actuated, broadband THz switch which transitions from a transparent state with low reflectivity, to an absorptive state for which both the reflectivity and transmission are strongly suppressed. Our device consists of a patterned high-resistivity silicon metamaterial layer that provides broadband reflection suppression by matching the impedance of free space. This is integrated with a VO₂ ground plane, which undergoes an IMT by means of a DC bias applied to an interdigitated electrode. THz time domain spectroscopy measurements reveal an active bandwidth of 700 GHz with suppressed reflection and more than 90% transmission amplitude modulation with a low insertion loss. We utilize finite-difference time domain (FDTD) simulations in order to examine the loss mechanisms of the device, as well as the sensitivity to polarization and incident angle. This device validates a general approach toward suppressing unwanted reflections in THz modulators and switches which can be easily adapted to a broad array of applications through straightforward modifications of the structural parameters.

Metamaterials provide a powerful platform to probe and enhance nonlinear responses in physical systems toward myriad applications. Herein, the development of a coupled nonlinear metamaterial (NLMM) featuring a self-adaptive response that selectively amplifies the magnetic field is reported. The resonance of the NLMM is suppressed in response to higher degrees of radio-frequency excitation strength and recovers during a subsequent low excitation strength phase, thereby exhibiting an intelligent, or nonlinear, behavior by passively sensing excitation signal strength and responding accordingly. The nonlinear response of the NLMM enables us to boost the signal-to-noise ratio during magnetic resonance imaging to an unprecedented degree. These results provide insights into a new paradigm to construct NLMMs consisting of coupled resonators and pave the way toward the utilization of NLMMs to address a host of practical technological applications.

Terahertz spectroscopy of the c-axis Josephson plasma resonance (JPR) in high-temperature cuprates is a powerful probe of superconductivity, providing a route to couple to and interact with the condensate. Electromagnetic coupling between metasurface arrays of split ring resonators (SRRs) and the JPR of a La_2−xSr_xCuO₄ single crystal (T_c = 32 K) is investigated. The metasurface resonance frequency (ω_MM), determined by the SRR geometry, is swept through the JPR frequency (ω_JPR = 1.53 THz) using a series of interchangeable tapes applied to the same single crystal. Terahertz reflectivity measurements on the resulting hybrid superconducting metamaterials (HSMMs) reveal anticrossing behavior characteristic of strong coupling. The experimental results, validated with numerical simulations, indicate a normalized Rabi frequency of Ω_R = 0.29. Further, it is shown that HSMMs with ω_MM > ω_JPR provide a route to couple to hyperbolic waveguide modes in c-axis cuprate samples. This work informs future possibilities for optimizing the coupling strength of HSMMs and investigating nonlinear superconductivity under high field terahertz excitation.

The effects of large external fields on semiconductor nanostructures could reveal much about field-induced shifting of electronic states and their dynamical responses and could enable electro-optic device applications that require large and rapid changes in optical properties. Studies of quasi-dc electric field modulation of quantum dot (QD) properties have been limited by electrostatic breakdown processes observed under high externally applied field levels. To circumvent this, here we apply ultrafast terahertz (THz) electric fields with switching times on the order of 1 ps. We show that a pulsed THz electric field, enhanced by a microslit field enhancement structure (FES), can strongly manipulate the optical absorption properties of a thin film of CdSe and CdSe–CdS core–shell QDs on the subpicosecond time scale with spectral shifts that span the visible to near-IR range. Numerical simulations using a semiempirical tight binding model show that the band gap of the QD film can be shifted by as much a 79 meV during these time scales. The results allow a basic understanding of the field-induced shifting of electronic levels and suggest electro-optic device applications.

The dispersion of graphene nanoparticles in a fluid can enhance the thermophysical properties of the base liquid. The physics behind such a phenomenon has yet to be uncovered in the community. In this work, thermal transport in a graphene–water mixture is studied by classical molecular dynamics simulations. Several factors including orientation angle, curvature, thermal rectification, temperature, and van der Waals interaction are investigated, and special attention is paid to the effect on thermal conductance across graphene–water interfaces. It is found that thermal conductance increases from 13.92 to 26.70 MW/m² K as the orientation angle is increased from 0° to 90°. When the curved graphene is introduced by altering the length to width ratio from 1.0 to 1.8, the thermal conductance is elevated. However, as the length to width ratio exceeds 1.8, such a trend does not continue due to the variation of the intrinsic thermal conductivity of graphene and the formation of the complex graphene–water interface. Even though the curved graphene introduces an asymmetric assembly, no thermal rectification effect is observed for diverse directions of heat flux. It is demonstrated that the enhancement of overall thermal conductance of nanofluids is ascribed to the interface thermal transport rather than the base liquid with increasing temperature. This correlation is suppressed in a hydrophilic interface due to the structural change of liquid layer adjacent to the interface.

Manipulating the phase of electromagnetic radiation is of importance for applications ranging from communication to imaging. Here, real-time reconfigurable phase response and group delay of a tunable terahertz metamaterial consisting of dual-layer broadside coupled split-ring resonators is demonstrated. Utilizing electrostatic comb-drive actuators, the metamaterial resonant frequency is tuned by changing the lateral distance between the two layers which modifies the transmission amplitude and phase spectrum. The phase modulation is approximately 180° in the vicinity of the resonant frequency. In addition, remarkable modulation in the group delay of transmitted pulses (from –7 to 3 ps) is evaluated based on the measured frequency response using the convolution method when the lateral distance is changed from 0 to 24 µ⁢m. A two-port resonator model, derived from coupled-mode theory and supported by finite-element full-wave simulations, reveals the underlying physics of the modulation. Specifically, the coupling factor between the two layers plays a critical role, the tuning of which provides a route for structure design and optimization. The capability of tuning the phase response and group delay enables applications, such as phase compensation and group-delay equalization at terahertz frequencies.

Diatoms are photosynthetic algae that exist ubiquitously throughout the planet in water environments. Over the preceding decades, the diatom exoskeletons, termed frustules, featuring abundant micro- and nanopores, have served as the source material and inspiration for myriad research efforts. In this work, it is demonstrated that frustule-inspired hierarchical nanostructure designs may be utilized in the fabrication of metamaterial absorbers, thereby realizing a broadband infrared (IR) absorber with excellent performance in terms of absorption. In an effort to investigate the origin of this absorption characteristic, numerical models are developed to study these structures, revealing that the hierarchical organization of the constituent nanoparticulate metamaterial unit cells introduce an additional resonance mode to the device, broadening the absorption spectrum. It is further demonstrated that the resonant peaks shift linearly as a function of inter-unit-cell spacing in the metamaterial, which is attributed to the induced collective dipole mode by the nanoparticles. Ultimately, the work herein represents an innovative perspective in terms of the design and fabrication of IR absorbers inspired by naturally occurring biomaterials, offering the potential to lead to advances in metamaterial absorber technology.

In this letter, we report optical pump terahertz (THz) near-field probe (n-OPTP) and optical pump THz near-field emission (n-OPTE) experiments of graphene/InAs heterostructures. Near-field imaging contrasts between graphene and InAs using these newly developed techniques as well as spectrally integrated THz nano-imaging (THz s-SNOM) are systematically studied. We demonstrate that in the near-field regime (λ/6000), a single layer of graphene is transparent to near-IR (800 nm) optical excitation and completely “screens” the photo-induced far-infrared (THz) dynamics in its substrate (InAs). Our work reveals unique frequency-selective ultrafast dynamics probed at the near field. It also provides strong evidence that n-OPTE nanoscopy yields contrast that distinguishes single-layer graphene from its substrate.

The plasmon resonances of nanostructures enable wide applications from highly sensitive sensing to high-resolution imaging, through the improvement of photogeneration rate stimulated by the local field enhancement. However, quantitative experimental studies on the localized heating and the thermal transport process in the vicinity of plasmonics are still lacking because of the diffraction limit in conventional optothermal methodologies. In this work, we demonstrate an approach based on Raman thermometry to probe the near-field heating caused by plasmonics. An array of Au nanoparticles (AuNPs) fabricated by the template-assisted method is used to generate the near field effect. Multi-walled carbon nanotubes (MWCNTs) dispersed on the AuNPs are employed to quantify the near-field heating from their Raman peak shifts. Results show that the temperature rise in MWCNTs on AuNPs is much higher than that in a control group under the same laser irradiation. Further analysis indicates that the enhanced photon absorption of MWCNTs attributed to plasmon resonances is partially responsible for the different heating effect. The nonuniform thermal hot spots at the nanoscale can result in the quasi-ballistic thermal transport of phonons in MWCNTs, which is another reason for the temperature rise. Our results can be used to understand plasmonic heating effects as well as to explore quasi-ballistic thermal transport in carbon-based low-dimensional materials by tailoring the geometry or size of plasmonic nanostructures.

Magnetic resonance imaging (MRI) represents a mainstay among the diagnostic imaging tools in modern healthcare. Signal-to-noise ratio (SNR) represents a fundamental performance metric of MRI, the improvement of which may be translated into increased image resolution or decreased scan time. Recently, efforts towards the application of metamaterials in MRI have reported improvements in SNR through their capacity to interact with electromagnetic radiation. While promising, the reported applications of metamaterials to MRI remain impractical and fail to realize the full potential of these unique materials. Here, we report the development of a magnetic metamaterial enabling a marked boost in radio frequency field strength, ultimately yielding a dramatic increase in the SNR (~ 4.2×) of MRI. The application of the reported magnetic metamaterials in MRI has the potential for rapid clinical translation, offering marked enhancements in SNR, image resolution, and scan efficiency, thereby leading to an evolution of this diagnostic tool.

In the past few decades, electromagnetic metamaterial absorbers have attracted tremendous attention due to near unity absorption of incident electromagnetic waves over a desired frequency range determined by the metamaterial inclusions as opposed to the constituent material properties. Importantly, metamaterial absorbers enable numerous potential applications which include wave manipulation, terahertz and infrared imaging, energy harvesting, radiative cooling, and chemical detection. To understand the underlying physics of metamaterial absorbers, various theoretical models have been developed. However, these models are seemingly conceptually unrelated, each yielding a distinct set of equations and conclusions. This paper reviews four prevalent theoretical approaches which include effective medium theory, transmission line modelling, coupled mode theory, and interference theory. We show that each of the four theoretical approaches provides an understanding of metamaterial absorbers from different points-of-view, each with distinct advantages and limitations. Moreover, the four theoretical models are interconnected and we discuss that, quite generally, impedance matching is the crucial condition for perfect absorption.

Electromagnetic metamaterials, which are a major type of artificially engineered materials, have boosted the development of optical and photonic devices due to their unprecedented and controllable effective properties, including electric permittivity and magnetic permeability. Metamaterials consist of arrays of subwavelength unit cells, which are also known as meta-atoms. Importantly, the effective properties of metamaterials are mainly determined by the geometry of the constituting subwavelength unit cells rather than their chemical composition, enabling versatile designs of their electromagnetic properties. Recent research has mainly focused on reconfigurable, tunable, and nonlinear metamaterials towards the development of metamaterial devices, namely, metadevices, via integrating actuation mechanisms and quantum materials with meta-atoms. Microelectromechanical systems (MEMS), or microsystems, provide powerful platforms for the manipulation of the effective properties of metamaterials and the integration of abundant functions with metamaterials. In this review, we will introduce the fundamentals of metamaterials, approaches to integrate MEMS with metamaterials, functional metadevices from the synergy, and outlooks for metamaterial-enabled photonic devices.

Terahertz perfect absorbers represent an essential photonic component for detecting, modulating, and manipulating terahertz radiation. We utilize single-layer H-shaped all-silicon arrays to demonstrate tunable ultra-broadband terahertz wave absorption. Experiment and simulation reveal near unity absorption at 1 THz, with a bandwidth of ~913 GHz for ≥90% absorbance. The absorption is optically tunable, exhibiting a resonance frequency blueshift by 420 GHz, while the peak absorbance remains over 99%. The dynamic response upon optical excitation depends on the penetration depth of the pump beam in silicon, as demonstrated through simulations that take into account the depth dependence of the carrier concentration in the all-silicon metamaterial perfect absorber. Notably, our all-silicon and ultrabroadband metamaterial perfect absorber is compatible with CMOS processing, potentially facilitating the development of terahertz detectors. Furthermore, the demonstrated tunable response may find potential applications toward creating dynamic functional terahertz devices, such as modulators and switches.

A typical metamaterial perfect absorber (MPA) is comprised of a metamaterial layer, a dielectric spacer, and a ground plane. The conventional spacer material is usually a lossy dielectric with little-dispersion for the purpose of easing the design and optimization procedure of the MPA. In this paper, we present the design, fabrication, and characterization of metamaterial perfect absorbers with a highly dispersive spacer, which is compatible with functional microelectromechanical systems. The measured dispersive permittivity of a silicon nitride thin film is used in modeling the absorption response of MPAs with rigorous coupled wave analysis. Different designs of MPA structures are fabricated and characterized. Spectroscopy data shows two perfect absorption peaks in wavelengths ranging from 8 µm to 20 µm, which supports the theoretical calculation and numerical simulation. The dispersion of silicon nitride enables the shared resonant modes of the two peak wavelengths and decreases the wavelength shift led by variations in structural parameters. We demonstrate that the use of dispersive dielectric materials in MPAs potentiates various functional devices.

Recently, with advances in acoustic metamaterial science, the possibility of sound attenuation using subwavelength structures, while maintaining permeability to air, has been demonstrated. However, the ongoing challenge addressed herein is the fact that among such air-permeable structures to date, the open area represents only small fraction of the overall area of the material. In the presented paper in order to address this challenge, we first demonstrate that a transversely placed bilayer medium with large degrees of contrast in the layers’ acoustic properties exhibits an asymmetric transmission, similar to the Fano-like interference phenomenon. Next, we utilize this design methodology and propose a deep-subwavelength acoustic metasurface unit cell comprising nearly 60% open area for air passage, while serving as a high-performance selective sound silencer. Finally, the proposed unit-cell performance is validated experimentally, demonstrating a reduction in the transmitted acoustic energy of up to 94%. This ultra-open metamaterial design, leveraging a Fano-like interference, enables high-performance sound silencing in a design featuring a large degree of open area, which may find utility in applications in which highly efficient, air-permeable sound silencers are required, such as smart sound barriers, fan or engine noise reduction, among others.

Dynamic fracture of borosilicate glass through focusing of high-amplitude nanosecond surface acoustic waves (SAWs) at the micron scale is investigated in an all-optical experiment. SAWs are generated by a picosecond laser excitation pulse focused into a ring-shaped spot on the sample surface. Interferometric images capture the SAW as it converges towards the center, focuses, and subsequently diverges. Above a laser energy threshold, damage at the acoustic focal point is observed. Numerical calculations help us determine the time evolution of the stress distribution. We find that the glass withstands a local tensile stress of at least 6 GPa without fracture.

Diatom frustules are a type of porous silicon dioxide microparticle that has long been used in applications ranging from biomedical sensors to dye-sensitized solar cells. The favorable material properties, enormous surface area, and enhanced light scattering capacity support the promise of diatom frustules as candidates for next generation biomedical devices and energy applications. In this study, the vapor–liquid–solid (VLS) method is employed to incorporate silica nanowires on the surface of diatom frustules. Compared to the original frustule structures, the frustule–nanowire composite material’s surface area increases over 3-fold, and the light scattering ability increases by 10%. By varying the gold catalyst thickness during the VLS process, tuning of the resultant nanowire length/density is achieved. Through material characterization, it is determined that both float growth and root growth processes jointly result in the growth of the silica nanowires. From a thermodynamics point of view, the preferential growth of the silica nanowires on frustules is found to have resulted from the enormous partial surface area of gold nanoparticles on the diatom frustules. The frustule–nanowire composite materials have potential applications in the development of novel biomedical sensing devices and may greatly enhance next generation solar cell performance.

Central venous catheters (CVCs) are implanted in the majority of dialysis patients despite increased patient risk due to thrombotic occlusion and biofilm formation. Current solutions remain ineffective at preventing these complications and treatment options are limited and often harmful. We present further analysis of the previously proposed water infused surface protection (WISP) technology, an active method to reduce protein adsorption and effectively disrupt adsorbed protein sheaths on the inner surface of CVCs. A WISP CVC is modeled by a hollow fiber membrane (HFM) in a benchtop device which continuously infuses a saline solution across the membrane wall into the blood flow, creating a blood-free boundary layer at the lumen surface. Total protein adsorption is measured under various experimental conditions to further test WISP performance. The WISP device shows reduced protein adsorption as blood and WISP flow rates increase (P < 0.040) with up to a 96% reduction in adsorption over the no WISP condition. When heparin is added to the WISP flow, protein adsorption (0.097[+0.035/-0.055] µg/mm²) is reduced when compared to both bolus administration and nondoped WISP, 0.406(+0.056/-0.065) µg/mm² (P = 0.001) and 0.191 (+0.076/-0.126) (P = 0.029), respectively. Additionally, when heparinized WISP is applied to a preadsorbed protein layer, 0.375(+0.114/-0.164) µg/mm², it displays the ability to reduce the previously-adsorbed protein, 0.186((+0.058/-0.084) µg/mm² (P = 0.0012), suggesting aptitude for intermittent treatments. The WISP technology not only shows the ability to reduce protein adsorption, but also the ability to remove preadsorbed material by effectively delivering drugs to the point of adsorption; functionalities that could greatly improve clinical outcomes.

Metamaterial absorbers are typically comprised of a layer of split-ring resonators and a ground plane with a dielectric spacer layer that provides structural support and in which absorbed energy is deposited. We address the question “What happens to the absorption if the spacer layer is removed?” through the design, fabrication, and characterization of a terahertz metamaterial absorber with air as the spacer layer. Reflection based terahertz time-domain spectroscopy was employed to measure the absorption and interference theory was used to interpret the results. The surface current in the gold ground plane and split-ring resonator layer is solely responsible for the absorption in the form of joule heating. In comparison to dielectric spacer layer absorbers, the quality factor is increased by a factor of ~3. The electric field is highly concentrated in the volume between split-ring resonator layer and the ground plane offering the potential for novel sensing application if materials can be incorporated into this region (e.g. with microfluidics). In the spirit of this possibility, simulations of the absorption have been performed. The variation of the real part of the permittivity of the spacer material results in an absorption peak frequency shift, while a change in the imaginary part affects the quality factor and amplitude. Ultimately, the high quality factor and the absence of the spacer material provide the air-spacer metamaterial absorber with unique advantages for sensing applications.

We present a comprehensive investigation of a continuously tunable metamaterial perfect absorber operating at terahertz frequencies. In particular, we investigate a three-layer absorber structure consisting of a layer of split ring resonators and a metallic ground plane, with a central layer consisting of a mechanically tunable air-spaced layer. The absorber was characterized using terahertz time-domain spectroscopy in reflection (at normal incidence) as a function of spacer thickness from 0 to 1000 µm. Our experimental measurements reveal the detailed evolution of the absorption bands as a function of spacing, in excellent agreement with analysis using interference theory and simulation. Our Fabry-Pérot-like structure provides an avenue for achieving massive tunability in metamaterial absorber devices.

In this work, a time-domain fluorescence spectroscopy technique is developed to characterize thermophysical properties of polymers. The method is based on fluorescence thermometry of materials under periodic pulse heating. In the characterization, a continuous laser (405 nm) is modulated with adjustable periodic heating and fluorescence excitation. The temperature rise at sample surface due to laser heating is probed from simultaneous fluorescence spectrum. Thermal diffusivity can be determined from the relationship between normalized temperature rise and the duration of laser heating. To verify this technique, thermal diffusivity of a polymer material (PVC) is characterized as 1.031 × 10^-7 m²/s, agreeing well with reference data. Meanwhile, thermal conductivity can be obtained by the hot plate method. Then, both steady and unsteady thermophysical properties are available. Quenching effect of fluorescence signal in our measurement can be ignored, as validated by longtime laser heating experiments. The uncertainty induced by uniformity of laser heating is negligible as analyzed through numerical simulations. This non-destructive fluorescence-based technique does not require exact value about laser absorption and calibration experiment for temperature coefficient of fluorescence signals. Considering that most polymers can excite sound fluorescence signal, this method can be well applied to thermal characterization of polymer-based film or bulk materials.

Acoustic metasurfaces represent a family of planar wavefront-shaping devices garnering increasing attention due to their capacity for novel acoustic wave manipulation. By precisely tailoring the geometry of these engineered surfaces, the effective refractive index may be modulated and, consequently, acoustic phase delays tuned. Despite the successful demonstration of phase engineering using metasurfaces, amplitude modulation remains overlooked. Herein, we present a class of metasurfaces featuring a horn-like space-coiling structure, enabling acoustic control with simultaneous phase and amplitude modulation. The functionality of this class of metasurfaces, featuring a gradient in channel spacing, has been investigated theoretically and numerically and an equivalent model simplifying the structural behavior is presented. A metasurface featuring this geometry has been designed and its functionality in modifying acoustic radiation patterns experimentally validated. This class of acoustic metasurface provides an efficient design methodology enabling complete acoustic wave manipulation, which may find utility in applications including biomedical imaging, acoustic communication, and non-destructive testing.

Dynamic polarization control of light is essential for numerous applications ranging from enhanced imaging to material characterization and identification. We present a reconfigurable terahertz metasurface quarter-wave plate consisting of electromechanically actuated microcantilever arrays. Our anisotropic metasurface enables tunable polarization conversion through cantilever actuation. Specifically, voltage-based actuation provides mode-selective control of the resonance frequency, enabling real-time tuning of the polarization state of the transmitted light. The polarization tunable metasurface has been fabricated using surface micromachining and characterized using terahertz time domain spectroscopy. We observe a ~230 GHz cantilever actuated frequency shift of the resonance mode, sufficient to modulate the transmitted wave from pure circular polarization to linear polarization. Our CMOS-compatible tunable quarter-wave plate enriches the library of terahertz optical components, thereby facilitating practical applications of terahertz technologies.

Metamaterial absorbers typically consist of a metamaterial layer, a dielectric spacer layer, and a metallic ground plane. We have investigated the dependence of the metamaterial absorption maxima on the spacer layer thickness and the reflection coefficient of the metamaterial layer obtained in the absence of the ground plane layer. Specifically, we employ interference theory to obtain an analytical expression for the spacer thickness needed to maximize the absorption at a given frequency. The efficacy of this simple expression is experimentally verified at terahertz frequencies through detailed measurements of the absorption spectra of a series of metamaterials structures with different spacer thicknesses. Using an array of split-ring resonators (SRRs) as the metamaterial layer and SU8 as the spacer material we observe that the absorption peaks redshift as the spacer thickness is increased, in excellent agreement with our analysis. Our findings can be applied to guide metamaterial absorber designs and understand the absorption peak frequency shift of sensors based on metamaterial absorbers.

The terahertz (THz) dielectric properties of super-aligned multi-walled carbon nanotube (MWCNT) films were characterized in the frequency range from 0.1 to 2.5 THz with terahertz time-domain spectroscopy. The refractive index, effective permittivity, and conductivity were retrieved from the measured transmission spectra with THz incident wave polarized parallel and perpendicular to the orientation of carbon nanotubes (CNTs), and a high degree of polarization dependence was observed. The Drude–Lorentz model combined with Maxwell-Garnett effective medium theory was employed to explain the experimental results, revealing an obvious metallic behavior of the MWCNT films. Moreover, rectangular aperture arrays were patterned on the super-aligned MWCNT films with laser-machining techniques, and the transmission measurement demonstrated an extraordinarily enhanced transmission characteristic of the samples with incident wave polarized parallel to the orientation of the CNTs. Surface plasmon polaritons were employed to explain the extraordinarily enhanced transmission with high accuracy, and multi-order Fano profile was applied to model the transmission spectra. A high degree of agreement was exhibited among the experimental, numerical, and theoretical results.

We present a detailed analysis of the conditions that result in unity absorption in metamaterial absorbers to guide the design and optimization of this important class of functional electromagnetic composites. Multilayer absorbers consisting of a metamaterial layer, dielectric spacer, and ground plane are specifically considered. Using interference theory, the dielectric spacer thickness and resonant frequency for unity absorption can be numerically determined from the functional dependence of the relative phase shift of the total reflection. Further, using transmission line theory in combination with interference theory we obtain analytical expressions for the unity absorption resonance frequency and corresponding spacer layer thickness in terms of the bare resonant frequency of the metamaterial layer and metallic and dielectric losses within the absorber structure. These simple expressions reveal a redshift of the unity absorption frequency with increasing loss that, in turn, necessitates an increase in the thickness of the dielectric spacer. The results of our analysis are experimentally confirmed by performing reflection-based terahertz time-domain spectroscopy on fabricated absorber structures covering a range of dielectric spacer thicknesses with careful control of the loss accomplished through water absorption in a semiporous polyimide dielectric spacer. Our findings can be widely applied to guide the design and optimization of the metamaterial absorbers and sensors.

Protein adhesion in central venous catheters (CVCs) leads to fibrin sheath formation, the precursor to thrombotic and biofilm-related CVC failures. Advances in material properties and surface coatings do not completely prevent fibrin sheath formation and post-formation treatment options are limited and expensive. We propose water infused surface protection (WISP), an active method for prevention of fibrin sheath formation on CVCs, which creates a blood-free boundary layer on the inner surface of the CVC, limiting blood contact with the CVC lumen wall. A hollow fiber membrane (HFM) in a benchtop device served as a CVC testing model to demonstrate the WISP concept. Porcine blood was pumped through the HFM while phosphate buffered saline (PBS) was infused through the HFM wall, creating the WISP boundary layer. Protein adherences on model CVC surfaces were measured and imaged. Analytical and finite volume lubrication models were used to justify the assumption of a blood-free boundary layer. We found a 92.2% reduction in average adherent protein density when WISP is used, compared with our model CVC without WISP flow. Lubrication models matched our experimental pressure drop measurements suggesting that a blood-free boundary layer was created. The WISP technique also provides a novel strategy for drug administration for biofilm treatment. Reduction in adherent protein indicates a restriction on long-term fibrin sheath and biofilm formation making WISP a promising technology which improves a wide range of vascular access treatments.

A double layer spiral antenna with side length of 740 µm was fabricated by a multilayer electroplating process and bonded with an radio frequency identification chip by silver epoxy to form a microsensor chip. A theoretical power transfer model was built to optimize the power transfer efficiency. The resonant frequency of the microsensor was characterized inside a small coupling loop, exhibiting a high degree of agreement with theoretical results. A magnetically coupled communication and charging platform was developed to work with the microsensors. The reader antenna was composed of a coupling loop and a secondary coil with 40-mm diameter wrapped around a polycarbonate tube. To maximize the magnetic field generated inside the secondary coil, a lump circuit model was built and its resonant modes were analyzed. The maximum current inside the secondary coil was achieved at the serial resonant frequency, at which the current followed a sinusoidal distribution along the coil. The magnetic field distribution inside the coil was calculated to analyze the read-out of the reader antenna. The communication and power transfer was demonstrated with the microsensors flowing through the reader antenna by successfully retrieving the sensor ID.

Optical properties of colloidal semiconductor quantum dots (QDs), arising from quantum mechanical confinement of charge, present a versatile testbed for the study of how high electric fields affect the electronic structure of nanostructured solids. Studies of quasi-DC electric field modulation of QD properties have been limited by electrostatic breakdown processes under high externally applied electric fields, which have restricted the range of modulation of QD properties. In contrast, here we drive CdSe–CdS core–shell QD films with high-field THz-frequency electromagnetic pulses whose duration is only a few picoseconds. Surprisingly, in response to the THz excitation, we observe QD luminescence even in the absence of an external charge source. Our experiments show that QD luminescence is associated with a remarkably high and rapid modulation of the QD bandgap, which changes by more than 0.5 eV (corresponding to 25% of the unperturbed bandgap energy). We show that these colossal energy shifts can be explained by the quantum confined Stark effect even though we are far outside the regime of small field-induced shifts in electronic energy levels. Our results demonstrate a route to extreme modulation of material properties and to a compact, high-bandwidth THz detector that operates at room temperature.

We present a three-dimensional terahertz metamaterial perfect absorber (MPA) that exhibits a high quality factor and is polarization insensitive. The unit cell is composed of two orthogonally oriented copper stand-up split ring resonators deposited on a copper ground plane with capacitive gaps in free space away from the substrate. Near unity (99.6%) absorption at ~1.65 THz is experimentally obtained in excellent agreement with simulation results. The quality factor is ~37, which is quite large for a terahertz MPA because of reduced material losses in the all-metal structure. According to simulation results, the MPA is insensitive to the polarization of the incident wave, and more than 90% absorption can be achieved for angles of incidence up to 60° for both TE and TM polarized incident THz waves.

Diatoms are unicellular, photosynthetic algae that are ubiquitous in aquatic environments. Their unique, three-dimensional (3D) structured silica exoskeletons, also known as frustules, have drawn attention from a variety of research fields due to their extraordinary mechanical properties, enormous surface area, and unique optical properties. Despite their promising use in a range of applications, without methods to uniformly control the frustules’ alignment/orientation, their full potential in technology development cannot be realized. In this paper, we realized and subsequently modeled a simple bubbling method for achieving large-area, uniformly oriented Coscinodiscus species diatom frustules. With the aid of bubble-induced agitations, close-packed frustule monolayers were achieved on the water–air interface with up to nearly 90% of frustules achieving uniform orientation. The interactions between bubble-induced agitations were modeled and analyzed, demonstrating frustule submersion and an adjustment of the orientation during the subsequent rise towards the water’s surface to be fundamental to the experimentally observed uniformity. The method described in this study holds great potential for frustules’ engineering applications in a variety of technologies, from sensors to energy-harvesting devices.

We present a superconducting metamaterial saturable absorber at terahertz frequencies. The metamaterial was designed to have a resonant absorption peak at 0.5 THz for T = 10 K. The absorber consists of an array of split ring resonators (SRRs) etched from a 100 nm YBa₂Cu₃O₇ film. A polyimide spacer layer and gold ground plane are placed above the SRRs using the metamaterial tape concept, creating a reflecting perfect absorber. Increasing either the temperature or incident electric field (E) decreases the superconducting condensate density and corresponding kinetic inductance of the SRRs. This alters the impedance matching in the metamaterial, broadening the resonance and reducing the peak absorption. At low electric fields, the experimental absorption was optimized near 80% at f = 0.47 THz for T = 10 K and decreased to 20% for T = 70 K. For E = 40 kV/cm and T = 10 K, the peak absorption was 70%, decreasing to 40% at 200 kV/cm, corresponding to a modulation of 43%.

A double layer spiral antenna with side length of 380 µm was fabricated by a multi-step electroplating process, and integrated with a commercialized passive RFID chip to realize the RF power harvesting and communication functions of a microsensor. To power up and communicate with the microchips, a single layer spiral reader antenna was fabricated on top of a glass substrate with side length of 1 mm. The microchips and the reader antenna were both optimized at the frequency of 915 MHz. Due to the small size of the reader antenna, the strength of the magnetic field decreased dramatically along the axial direction of the reader antenna, which limited the working distance to within 1 mm. To enclose the microchips within the reading range, a three-layer microfluidic channel was designed and fabricated. The channel and cover layers were fabricated by laser cutting of acrylic sheets, and bonded with the glass substrate to form the channel. To operate multiple microchips simultaneously, separation and focusing function units were also designed. Low loss pump oil was used to transport the microchips flowing inside the channel. Within the reading area, the microchips were powered up, and their ID information was retrieved and displayed on the computer interface successfully.

We experimentally demonstrate the extraordinary transmission of THz waves through super-aligned multi-walled carbon nanotube (MWCNT) films with one-dimensional arrays of sub-wavelength rectangular gratings in the broad frequency range from 0.2 to 2.5 THz. To achieve this, two kinds of MWCNT films (1 µm and 3 µm in thickness) were fabricated by drawing from a sidewall of super-aligned nanotube arrays synthesized by low pressure chemical vapor deposition. The measured complex refraction index of the film exhibits highly anisotropic transmission of THz waves through the MWCNTs. The anisotropy depends not only on the polarization direction of the THz waves but also on the orientation of the MWCNT gratings. We found that the resonantly extraordinary THz transmission originated from the surface plasmon polaritons supported by periodically patterned carbon nanotube gratings. Our experimental results may provide important insights for emerging THz plasmonic devices based on carbon nanotubes.

This paper presents the design, fabrication, and characterization of a real-time voltage-tunable terahertz metamaterial based on microelectromechanical systems and broadside-coupled split-ring resonators. In our metamaterial, the magnetic and electric interactions between the coupled resonators are modulated by a comb-drive actuator, which provides continuous lateral shifting between the coupled resonators by up to 20 µm. For these strongly coupled split-ring resonators, both a symmetric mode and an anti-symmetric mode are observed. With increasing lateral shift, the electromagnetic interactions between the split-ring resonators weaken, resulting in frequency shifting of the resonant modes. Over the entire lateral shift range, the symmetric mode blueshifts by ~60 GHz, and the anti-symmetric mode redshifts by ~50 GHz. The amplitude of the transmission at 1.03 THz is modulated by 74%; moreover, a 180° phase shift is achieved at 1.08 THz. Our tunable metamaterial device has myriad potential applications, including terahertz spatial light modulation, phase modulation, and chemical sensing. Furthermore, the scheme that we have implemented can be scaled to operate at other frequencies, thereby enabling a wide range of distinct applications.

We investigate the nonlinear response of terahertz (THz) metamaterial perfect absorbers consisting of electric split ring resonators on GaAs integrated with a polyimide spacer and gold ground plane. These perfect absorbers on bulk semi-insulating GaAs are characterized using high-field THz time-domain spectroscopy. The resonance frequency redshifts 20 GHz and the absorbance is reduced by 30% as the incident peak field is increased from 30 to 300 kV/cm. The nonlinear response arises from THz field driven interband transitions and intervalley scattering in the GaAs. To eliminate the Fresnel losses from the GaAs substrate, we design and fabricate a flexible metamaterial saturable perfect absorber. The ability to create nonlinear absorbers enables appealing applications such as optical limiting and self-focusing.

The development of responsive metamaterials has enabled the realization of compact tunable photonic devices capable of manipulating the amplitude, polarization, wave vector and frequency of light. Integration of semiconductors into the active regions of metallic resonators is a proven approach for creating nonlinear metamaterials through optoelectronic control of the semiconductor carrier density. Metal-free subwavelength resonant semiconductor structures offer an alternative approach to create dynamic metamaterials. We present InAs plasmonic disk arrays as a viable resonant metamaterial at terahertz frequencies. Importantly, InAs plasmonic disks exhibit a strong nonlinear response arising from electric field-induced intervalley scattering, resulting in a reduced carrier mobility thereby damping the plasmonic response. We demonstrate nonlinear perfect absorbers configured as either optical limiters or saturable absorbers, including flexible nonlinear absorbers achieved by transferring the disks to polyimide films. Nonlinear plasmonic metamaterials show potential for use in ultrafast terahertz (THz) optics and for passive protection of sensitive electromagnetic devices.

Transmission spectra of terahertz waves through a two-dimensional array of asymmetric rectangular apertures on super-aligned multi-walled carbon nanotube films were obtained experimentally. In this way, the anisotropic transmission phenomena of carbon nanotube films were observed. For a terahertz wave polarization parallel to the orientation of the carbon nanotubes and along the aperture short axis, sharp resonances were observed and the resonance frequencies coincided well with the surface plasmon polariton theory. In addition, the minima of the transmission spectra were in agreement with the location predicted by the theory of Wood’s anomalies. Furthermore, it was found that the resonance profiles through the carbon nanotube films could be well described by the Fano model.

Meta-liquid crystals, a novel form of tunable 3D metamaterials, are proposed and experimentally demonstrated in the terahertz frequency regime. A morphology change under a bias electric field and a strong modulation of the transmission are observed. In comparison to conventional liquid crystals, there is considerable freedom to prescribe the electromagnetic properties through the judicious design of the meta-atom geometry.

We use intense terahertz pulses to excite the resonant mode (0.6 THz) of a micro-fabricated dipole antenna with a vacuum gap. The dipole antenna structure enhances the peak amplitude of the in-gap THz electric field by a factor of ~170. Above an in-gap E-field threshold amplitude of ~10 MV/cm⁻¹, THz-induced field electron emission is observed as indicated by the field-induced electric current across the dipole antenna gap. Field emission occurs within a fraction of the driving THz period. Our analysis of the current (I) and incident electric field (E) is in agreement with a Millikan-Lauritsen analysis where log (I) exhibits a linear dependence on 1/E. Numerical estimates indicate that the electrons are accelerated to a value of approximately one tenth of the speed of light.

Diatomite earth, preferably maintained the morphology and structure of nature diatom, with the characteristic of unique multi-level nanopores and large specific surface area. Diatom is a low-cost at large quantity through repeatable separation from diatomite earth, suitable for industrial application. In this study, the disk-shaped diatom shell particles, size distribution between 40-80 µm, were extracted out by physical settlement method and mechanical screening method. Then, according to the particles self-assembly close-packed principles at air-water interface, the fast close-packed feasibility of hydrophobic diatom shells in a large area was studied. The results showed that by controlling the concentration and forms of diatom shell particles, centimeter level close-packed monolayer can be obtained. The close-packed monolayer area of floating diatom shell particles was over 23.7 cm², and on drying substrate was about 5.0 cm².

The development of a biologically enabled micro- and nanostencil lithography approach using diatoms is demonstrated. Diatom frustules are initially purified, sorted, and aligned into compact monolayers on underlying silicon substrates. Subsequently, the diatom monolayers are employed as shadow masks during the electron beam deposition of gold (Au) thin films, a process which enables the capacity to mirror the intricate micro- and nanoporous frustule architecture on the underlying silicon substrates. Following Au deposition and diatom frustule dissolution, both sub-micron and nanoscale gold patterns on silicon are realized using this approach. This unique method yields the highly structured patterning of gold and other materials on a variety of substrates, with feature sizes ranging from the sub-micron to the nanoscale, enabling a host of diverse applications.

Over the past decade, breakthroughs in the generation and control of ultrafast high-field terahertz (THz) radiation have led to new spectroscopic methodologies for the study of light-matter interactions in the strong-field limit. In this review, we will outline recent experimental demonstrations of non-linear THz material responses in materials ranging from molecular gases, to liquids, to varieties of solids – including semiconductors, nanocarbon, and correlated electron materials. New insights into how strong THz fields interact with matter will be discussed in which a THz field can act as either a non-resonant electric field or a broad bandwidth pulse driving specific resonances within it. As an emerging field, non-linear THz spectroscopy shows promise for elucidating dynamic problems associated with next generation electronics and optoelectronics, as well as for demonstrating control over collective material degrees of freedom.

We present our recent progress on a highly flexible tunable perfect absorber at terahertz frequencies. Metamaterial unit cells were patterned on thin GaAs patches, which were fashioned in an array on a 10 µm thick polyimide substrate via semiconductor transfer technique, and the backside of the substrate was coated with gold film as a ground plane. Optical-pump THz-probe reflection measurements show that the absorptivity can be tuned up to 25% at 0.78 THz and 40% at 1.75 THz through photo-excitation of free carriers in GaAs layers in presence of 800 nm pump beam. Our flexible tunable metamaterial perfect absorber has potential applications in energy harvesting, THz modulation and even camouflages coating.

We explored the use of the optically transparent semiconductor indium tin oxide (ITO) as an alternative to optically opaque metals for the fabrication of photonic structures in terahertz (THz) near-field studies. Using the polaritonics platform, we confirmed the ability to clearly image both bound and leaky electric fields underneath an ITO layer. We observed good agreement between measured waveguide dispersion and analytical theory of an asymmetric metal-clad planar waveguide with TE and TM polarizations. Further characterization of the ITO revealed that even moderately conductive samples provided sufficiently high quality factors for studying guided and leaky wave behaviors in individual transparent THz resonant structures such as antennas or split ring resonators. However, without higher conductive ITO, the limited reflection efficiency and high radiation damping measured here both diminish the applicability of ITO for high-reflecting, arrayed, or long path-length elements.

The mechanical properties of the local microenvironment may have important influence on the fate and function of adult tissue progenitor cells, altering the regenerative process. This is particularly critical following a myocardial infarction, in which the normal, compliant myocardial tissue is replaced with fibrotic, stiff scar tissue. In this study, we examined the effects of matrix stiffness on adult cardiac side population (CSP) progenitor cell behavior. Ovine and murine CSP cells were isolated and cultured on polydimethylsiloxane substrates, replicating the elastic moduli of normal and fibrotic myocardium. Proliferation capacity and cell cycling were increased in CSP cells cultured on the stiff substrate with an associated reduction in cardiomyogeneic differentiation and accelerated cell ageing. In addition, culture on stiff substrate stimulated upregulation of extracellular matrix and adhesion proteins gene expression in CSP cells. Collectively, we demonstrate that microenvironment properties, including matrix stiffness, play a critical role in regulating progenitor cell functions of endogenous resident CSP cells. Understanding the effects of the tissue microenvironment on resident cardiac progenitor cells is a critical step toward achieving functional cardiac regeneration.

We present a microfluidic read-out platform concept for miniaturized radio-frequency identification (RFID) tags that will help identifying natural underground sources. Designed microfluidic platform has embedded spiral reader antennas for data communication via near field magnetic coupling. To enable near field magnetic coupling with the reader antennas at reasonably low frequencies, we developed electrically ultrasmall (380 µm × 380 µm × 15 µm with resonance ~0.75 GHz) on-chip antennas. Concept feasibility was proven by a set of experiments. First, data communication between an RFID reader and a printed circuit board-level passive tag was demonstrated through miniaturized antennas. Second, coupling between passive reader and on-chip antennas was shown inside the fluidic environment. In addition, a chip separation mechanism was introduced to the platform to prevent cross-coupling events between RFID tags.

Development of tunable, dynamic, and broad bandwidth metamaterial designs is a keystone objective for metamaterials research, necessary for the future viability of metamaterial optics and devices across the electromagnetic spectrum. Yet, overcoming the inherently localized, narrow bandwidth, and static response of resonant metamaterials continues to be a challenging endeavor. Resonant perfect absorbers have flourished as one of the most promising metamaterial devices with applications ranging from power harvesting to terahertz imaging. Here, an optically modulated resonant perfect absorber is presented. Utilizing photo-excited free carriers in silicon pads placed in the capacitive gaps of split ring resonators, a dynamically modulated perfect absorber is designed and fabricated to operate in reflection. Large modulation depth (38% and 91%) in two absorption bands (with 97% and 92% peak absorption) is demonstrated, which correspond to the LC (0.7 THz) and dipole (1.1 THz) modes of the split ring resonators.

The design of artificial nonlinear materials requires control over internal resonant charge densities and local electric field distributions. We present a MM design with a structurally controllable oscillator strength and local electric field enhancement at terahertz frequencies. The MM consists of a split ring resonator (SRR) array stacked above an array of closed conducting rings. An in-plane, lateral shift of a half unit cell between the SRR and closed ring arrays results in an increase of the MM oscillator strength by a factor of 4 and a 40% change in the amplitude of the resonant electric field enhancement in the SRR capacitive gap. We use terahertz time-domain spectroscopy and numerical simulations to confirm our results. We show that the observed electromagnetic response in this MM is the result of image charges and currents induced in the closed rings by the SRR.

The mapping of traction forces is crucial to understanding the means by which cells regulate their behavior and physiological function to adapt to and communicate with their local microenvironment. To this end, polymeric micropillar arrays have been used for measuring cell traction force. However, the small scale of the micropillar deflections induced by cell traction forces results in highly inefficient force analyses using conventional optical approaches; in many cases, cell forces may be below the limits of detection achieved using conventional microscopy. To address these limitations, the moiré phenomenon has been leveraged as a visualization tool for cell force mapping due to its inherent magnification effect and capacity for whole-field force measurements. This Letter reports an optomechanical cell force sensor, namely, a double-sided micropillar array (DMPA) made of poly(dimethylsiloxane), on which one side is employed to support cultured living cells while the opposing side serves as a reference pattern for generating moiré patterns. The distance between the two sides, which is a crucial parameter influencing moiré pattern contrast, is predetermined during fabrication using theoretical calculations based on the Talbot effect that aim to optimize contrast. Herein, double-sided micropillar arrays were validated by mapping mouse embryo fibroblast contraction forces and the resulting force maps compared to conventional microscopy image analyses as the reference standard. The DMPA-based approach precludes the requirement for aligning two independent periodic substrates, improves moiré contrast, and enables efficient moiré pattern generation. Furthermore, the double-sided structure readily allows for the integration of moiré-based cell force mapping into microfabricated cell culture environments or lab-on-a-chip devices.

This paper focuses on investigating that the influence of O₂, CO₂ and H₂O on characteristics of autothermal reforming of methane in micro premix chamber on Ni catalysts. In addition, the effect of catalytic wall temperature on autothermal reforming reaction of methane under a certain ratio of CH₄/CO₂/H₂O/O₂ is simulated. The results indicate that appropriately increasing O₂ concentration can increase the conversion efficiency of CH₄, so does adding CO₂ or H₂O. The positive effect of O₂, CO₂ and H₂O is more pronounced at the higher temperature. The temperature range of 650–750 K is the important transitional region in the reactions of CH₄/O₂, CH₄/H₂O and CH₄/CO₂. It also gives a guide to the available range of parameters in the high efficiency reforming process of micro-reactor.

This paper reports novel MEMS and NEMS-based fabrication processes for biocompatible, hollow cylindrical ferromagnetic structures for potential use as contrast agents for magnetic resonance imaging (MRI). Compared to previous works on Ni-based cylindrical-nanoshells and Fe-based double-disk particles, biocompatibility and yield issues were strongly considered in this development of a simplified fabrication process incorporating iron oxide thin films. The novel, simplified fabrication processes developed herein yield robust, reproducible fabrication methodologies for the further development of this new class of MRI contrast agents. Specifically, both micron- and nano-scale hollow cylindrical agents were successfully fabricated, the size regimes of which enable a wide array of potential imaging applications. The use of top-down engineering approaches to MRI contrast agent design such as reported herein offers the capacity for multiplexed imaging which may dramatically potentiate the capabilities of MRI imaging.

This paper reports the fabrication and characterization of composite hydrogel particles composed of poly(ethylene glycol) diacrylate (PEG-DA)-based hydrogels and x-ray attenuating payloads. The top–down fabrication method employed herein is demonstrated to yield composite hydrogel particles of varying size and shape for use as computed tomography (CT) imaging contrast agents. Characterization of the materials properties of the PEG-DA hydrogels was undertaken, demonstrating tunable mechanical properties of composite hydrogels based on hydrogel composition and UV cross-linking time. Analyses of the leakage rates of a conventional iodine-based small molecular contrast agent as well as a nanoparticulate x-ray attenuating material from the PEG-DA hydrogels were undertaken. In contradistinction to clinically available iodinated CT contrast agents, as well as recently developed nanoparticulate CT contrast agents, the approach presented herein yields an engineering flexibility to the design of CT contrast agents which may be leveraged to optimize this class of agents to a wide array of specific imaging and sensing applications.

We present optically tunable metamaterials (MMs) on flexible polymer sheets operating at terahertz (THz) frequencies. The flexible MMs, consisting of electric split-ring resonators (eSRRs) on patterned GaAs patches, were fabricated on a thin polyimide layer using a transfer technique. Optical excitation of the GaAs patches modifies the metamaterial response. Our experimental results revealed that, with increasing fluence, a transmission modulation depth of ~ 60% was achieved at the LC resonant frequency of 0.98 THz. In addition, a similar modulation depth was obtained over a broad range from 1.1 to 1.8 THz. Numerical simulations agree with experiment and indicate efficient tuning of the effective permittivity of the MMs. Our flexible tunable device paves the way to create multilayer nonplanar tunable electromagnetic composites for nonlinear and multifunctional applications, including sensing, modulation, and energy harvesting.

Recently, a novel class of magnetic resonance imaging (MRI) contrast agents developed using top-down microfabrication approaches has been reported. To realize the full capacity of this potentially paradigm-shifting approach to MRI contrast agent design, the integration of biocompatible materials with tunable magnetic properties was sought. To this end, deposition techniques yielding iron oxide thin films with a large range of readily tunable saturation magnetic polarization were developed using reactive sputtering under various conditions. Following the characterization of their chemical compositions and crystalline structures, the iron oxide thin films were subsequently utilized in the fabrication of size and shape specific magnetic double-disk microparticles, yielding the advantages of this new class of MRI contrast agents, including multiplexing capability, diffusion-driven signal amplification, and functional imaging capacity. The integration of iron oxides into this class of fabricated contrast agents offers several distinct advantages, including biocompatibility and the additional degree of freedom in the design of these agents achieved by the tunability of the iron oxide thin film magnetism, both of which are critical features in further optimizing these agents.

Metamaterials research continues to bear fruit in the form of novel devices and optics across the electromagnetic spectrum. This is especially true in the gigahertz, terahertz, and near infrared frequencies. Metamaterials also continue to be one of the fastest growing subdisciplines of anisotropy research, with most notable metamaterial advances based on inherently anisotropic designs. Despite significant progress, many challenges remain before fully dynamic, broad bandwidth, and nonlinear metamaterial devices become truly viable. We review the study of near field interactions, or coupling, in metamaterials with a focus on how manipulation of interactions in metamaterials has helped overcome some of the largest obstacles toward tunable metamaterials, broad bandwidth metamaterials, nonlinear metamaterials, and metamaterial experimental techniques.

Silicon oxynitride (SiON) is an important material to fabricate micro-electro-mechanical system (MEMS) devices due to its composition-dependent tunability in electronic and mechanical properties. In this work, the SiON film with 41.45% silicon, 32.77% oxygen and 25.78% nitrogen content was deposited by RF magnetron sputtering. Two types of optimized micro-structures including micro-cantilevers and micro-rotating-fingers were designed and fabricated using MEMS surface micromachining technology. The micro-cantilever bending tests were conducted using a nanoindenter to characterize the Young’s modulus of the SiON film. Owing to the elimination of the residual stress effect on the micro-cantilever structure, higher accuracy in the Young’s modulus was achieved from this technique. With the information of Young’s modulus of the film, the residual stresses were characterized from the deflection of the micro-rotating-fingers. This structure was able to locally measure a large range of tensile or compressive residual stresses in a thin film with sufficient sensitivities. The results showed that the Young’s modulus of the SiON film was 122 GPa and the residual stresses of the SiON film were 327 MPa in the crystallographic orientation of the wafer and 334 MPa in the direction perpendicular to the crystallographic orientation, both in compression. This work presents a comprehensive methodology to measure the Young’s modulus and residual stresses of a thin film with improved accuracy, which is promising for applications in mechanical characterization of MEMS devices.

We investigate the electromagnetic response of asymmetric broadside coupled split-ring resonators (ABC-SRRs) as a function of the relative in-plane displacement between the two component SRRs. The asymmetry is defined as the difference in the capacitive gap widths (Δ⁢g) between the two resonators comprising a coupled unit. We characterize the response of ABC-SRRs both numerically and experimentally via terahertz time-domain spectroscopy. As with symmetric BC-SRRs (Δ⁢g=0 µm), a large redshift in the LC resonance is observed with increasing displacement, resulting from changes in the capacitive and inductive coupling. However, for ABC-SRRs, in-plane shifting between the two resonators by more than 0.375 L₀ (L₀ = SRR sidelength) results in a transition to a response with two resonant modes, associated with decoupling in the ABC-SRRs. For increasing Δg, the decoupling transition begins at the same relative shift (0.375 L₀), though with an increase in the oscillator strength of the new mode. This strongly contrasts with symmetric BC-SRRs, which present only one resonance for shifts up to 0.75 L₀. Since all BC-SRRs are effectively asymmetric when placed on a substrate, an understanding of ABC-SRR behavior is essential for a complete understanding of BC-SRR based metamaterials.

Models of reabsorptive barriers require both a means to provide realistic physiologic cues to and quantify transport across a layer of cells forming the barrier. Here we have topographically-patterned porous membranes with several user-defined pattern types. To demonstrate the utility of the patterned membranes, we selected one type of pattern and applied it to a membrane to serve as a cell culture support in a microfluidic model of a renal reabsorptive barrier. The topographic cues in the model resemble physiological cues found in vivo while the porous structure allows quantification of transport across the cell layer. Sub-micron surface topography generated via hot-embossing onto a track-etched polycarbonate membrane, fully replicated topographical features and preserved porous architecture. Pore size and shape were analyzed with SEM and image analysis to determine the effect of hot embossing on pore morphology. The membrane was assembled into a bilayer microfluidic device and a human kidney proximal tubule epithelial cell line (HK-2) and primary renal proximal tubule epithelial cells (RPTEC) were cultured to confluency on the membrane. Immunofluorescent staining of both cell types revealed protein expression indicative of the formation of a reabsorptive barrier responsive to mechanical stimulation: ZO-1 (tight junction), paxillin (focal adhesions) and acetylated α-tubulin (primary cilia). HK-2 and RPTEC aligned in the direction of ridge/groove topography of the membrane in the device, evidence that the device has mechanical control over cell response. This topographically-patterned porous membrane provides an in vitro platform on which to model reabsorptive barriers with meaningful applications for understanding biological transport phenomenon, underlying disease mechanisms, and drug toxicity.

We demonstrate nonlinear metamaterial split ring resonators (SRRs) on GaAs at terahertz frequencies. For SRRs on doped GaAs films, incident terahertz radiation with peak fields of ~20–160 kV/cm drives intervalley scattering. This reduces the carrier mobility and enhances the SRR LC response due to a conductivity decrease in the doped thin film. Above ~160 kV/cm, electric field enhancement within the SRR gaps leads to efficient impact ionization, increasing the carrier density and the conductivity which, in turn, suppresses the SRR resonance. We demonstrate an increase of up to 10 orders of magnitude in the carrier density in the SRR gaps on semi-insulating GaAs. Furthermore, we show that the effective permittivity can be swept from negative to positive values with an increasing terahertz field strength in the impact ionization regime, enabling new possibilities for nonlinear metamaterials.

A generic experimental procedure is presented in this work to enhance the electrical responses of polydimethylsiloxane (PDMS) through incorporation of conducting polymer nanowires, while maintaining the desirable mechanical flexibility of PDMS. The conducting polypyrrole (PPy) nanowires are synthesized using a template method. The dielectric constants of the composites are characterized by impedance measurements, and the effect of nanowire concentration is investigated by the percolation theory. Using a continuous hyperbolic tangent function, critical volume fraction is estimated to be 9.6 vol%, at which an 85-fold enhancement in the dielectric constants is achieved. The viscoelastic properties of the composites are characterized by the stress relaxation nanoindentation tests, and the effect of nanowire concentration on the elastic modulus of composites is found to deviate significantly from the Wang–Pyrz model at the critical volume fraction. The tunable multifunctionality of PDMS composites that possess significantly enhanced electrical and moderate viscoelastic responses is desirable for many sensing and actuation applications.

We present optically tunable magnetic three-dimensional (3D) metamaterials at terahertz (THz) frequencies which exhibit a tuning range of ~30% of the resonance frequency. This is accomplished by fabricating 3D array structures consisting of double-split-ring resonators (DSRRs) on silicon on sapphire, fabricated using multilayer electroplating. Photoexcitation of free carriers in the silicon within the capacitive region of the DSRR results in a redshift of the resonant frequency from 1.74 to 1.16 THz. The observed frequency shift leads to a transition from a magnetic-to-bianisotropic response as verified through electromagnetic simulations and parameter retrieval. Our approach extends dynamic metamaterial tuning to magnetic control, and may find applications in switching and modulation, polarization control, or tunable perfect absorbers.

Polydimethylsiloxane (PDMS) micropillar-based biotransducers are extensively used in cellular force measurements. The accuracy of these devices relies on the appropriate material characterization of PDMS and modeling to convert the micropillar deformations into the corresponding forces. Cellular contraction is often accompanied by oscillatory motion, the frequency of which ranges in several hertz. In this paper, we developed a methodology to calculate the cellular contraction forces in the frequency domain with improved accuracy. The contraction data were first expressed as a Fourier series. Subsequently, we measured the complex modulus of PDMS using a dynamic nanoindentation technique. An improved method for the measurement of complex modulus was developed with the use of a flat punch indenter. The instrument dynamics was characterized, and the full contact region was identified. By incorporating both the Fourier series of contraction data and the complex modulus function, the cellular contraction force was calculated by finite-element analysis (FEA). The difference between the Euler beam formula and the viscoelastic FEA was discussed. The methodology presented in this work is anticipated to benefit the material characterization of other soft polymers and complex biological behavior in the frequency domain.

Electron–electron interactions can render an otherwise conducting material insulating, with the insulator–metal phase transition in correlated-electron materials being the canonical macroscopic manifestation of the competition between charge-carrier itinerancy and localization. The transition can arise from underlying microscopic interactions among the charge, lattice, orbital and spin degrees of freedom, the complexity of which leads to multiple phase-transition pathways. For example, in many transition metal oxides, the insulator–metal transition has been achieved with external stimuli, including temperature, light, electric field, mechanical strain or magnetic field. Vanadium dioxide is particularly intriguing because both the lattice and on-site Coulomb repulsion contribute to the insulator-to-metal transition at 340 K. Thus, although the precise microscopic origin of the phase transition remains elusive, vanadium dioxide serves as a testbed for correlated-electron phase-transition dynamics. Here we report the observation of an insulator–metal transition in vanadium dioxide induced by a terahertz electric field. This is achieved using metamaterial-enhanced picosecond, high-field terahertz pulses to reduce the Coulomb-induced potential barrier for carrier transport. A nonlinear metamaterial response is observed through the phase transition, demonstrating that high-field terahertz pulses provide alternative pathways to induce collective electronic and structural rearrangements. The metamaterial resonators play a dual role, providing sub-wavelength field enhancement that locally drives the nonlinear response, and global sensitivity to the local changes, thereby enabling macroscopic observation of the dynamics. This methodology provides a powerful platform to investigate low-energy dynamics in condensed matter and, further, demonstrates that integration of metamaterials with complex matter is a viable pathway to realize functional nonlinear electromagnetic composites.

We report on direct measurements of the magnetic near-field of metamaterial split ring resonators at terahertz frequencies using a magnetic field sensitive material. Specifically, planar split ring resonators are fabricated on a single magneto-optically active terbium gallium garnet crystal. Normally incident terahertz radiation couples to the resonator inducing a magnetic dipole oscillating perpendicular to the crystal surface. Faraday rotation of the polarisation of a near-infrared probe beam directly measures the magnetic near-field with 100 femtosecond temporal resolution and (λ/200) spatial resolution. Numerical simulations suggest that the magnetic field can be enhanced in the plane of the resonator by as much as a factor of 200 compared to the incident field strength. Our results provide a route towards hybrid devices for dynamic magneto-active control of light such as isolators, and highlight the utility of split ring resonators as compact probes of magnetic phenomena in condensed matter.

We investigate the interaction between terahertz waves and resonant antennas with sub-cycle temporal and λ/100 spatial resolution. Depositing antennas on a LiNbO₃ waveguide enables non-invasive electro-optic imaging, quantitative field characterization, and direct measurement of field enhancement (up to 40-fold). The spectral response is determined over a bandwidth spanning from DC across multiple resonances, and distinct behavior is observed in the near- and far-field. The scaling of enhancement and resonant frequency with gap size and antenna length agrees well with simulations.

In the last decade, the development of metamaterials has led to exotic phenomena not shown in nature, including negative refractive index, invisibility cloaking and perfect absorption. To achieve these effects requires creating magnetically resonant subwavelength structures, since naturally occurring magnetism typically occurs at relatively low frequencies. In the far-infrared, or terahertz (THz), region of the electromagnetic spectrum, it is difficult to obtain a strong magnetic response from planar metamaterials at normal incidence. In this paper, multilayer electroplating is used to fabricate three-dimensional (3D) split-ring resonators that stand up out of plane. This enables the maximum coupling to the magnetic response at normal incidence. Characterization using THz time-domain spectroscopy indicates a strong magnetic resonance, and parameter extraction reveals a negative permeability from 1 to 1.3 THz with the minimal value of −2. The successful design, fabrication and characterization of 3D metamaterials provide opportunities to achieve different electromagnetic properties and novel devices in the THz range.

There is an increasing trend to incorporate silicon carbide (SiC) into silicon oxides to improve the mechanical properties, thermal stability, and chemical resistance. In this work the silicon oxycarbide (SiOC) films were deposited by RF magnetron co-sputtering from silicon dioxide and silicon carbide targets. Subsequently rapid thermal annealing was applied to the as-deposited films to tune the mechanical properties. Energy dispersive spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and ellipsometry were employed to characterize the compositions and microstructure of the films. The residual stress of the films was calculated from the film–substrate curvature measurement using Stoney’s equation. The film stress changed from compressive to tensile after annealing, and it generally increased with carbon contents. The Young’s modulus and hardness were investigated by the depth-sensing nanoindentation, which were found to increase with the carbon content and annealing temperature. A thorough microstructural analysis was conducted to investigate the effect of carbon content and annealing temperature on the mechanical properties of SiOC films.

Silicon-rich and rare-earth-doped nitride materials are promising candidates for silicon-compatible photonic sources. This work investigates the thermal conductivity and photoluminescence (PL) of light emitting samples fabricated with a range of excess silicon concentrations and annealing temperatures using time-domain picosecond thermoreflectance and time-resolved photoluminescence. A direct correlation between the thermal conductivity and photoluminescence dynamics is demonstrated, as well as a significant reduction of thermal conductivity upon incorporation of erbium ions. These findings highlight the role of annealing and stoichiometry control in the optimization of light emitting microstructures suitable for the demonstration of efficient Si-compatible light sources based on the silicon nitride platform.

We investigate the conditions under which single layer metamaterials may be described by bulk optical constants. Terahertz time domain spectroscopy is utilized to investigate two types of geometries, both with two different sizes of embedding dielectric—cubic and tetragonal unit cells. The tetragonal metamaterials are shown to yield layer dependent optical constants, whereas the cubic metamaterials yielded layer independent optical constants. We establish guidelines for when ε and μ can be used as material parameters for single layer metamaterials. Experimental results at terahertz frequencies are presented and supported by full wave three-dimensional electromagnetic simulations.

We have wrapped metallic cylinders with strongly absorbing metamaterials. These resonant structures, which are patterned on flexible substrates, smoothly coat the cylinder and give it an electromagnetic response designed to minimize its radar cross section. We compare the normal-incidence, small-beam reflection coefficient with the measurement of the far-field bistatic radar cross section of the sample, using a quasi-planar THz wave with a beam diameter significantly larger than the sample dimensions. In this geometry we demonstrate a near-400-fold reduction of the radar cross section at the design frequency of 0.87 THz. In addition we discuss the effect of finite sample dimensions and the spatial dependence of the reflection spectrum of the metamaterial.

Physiologically-representative and well-controlled in vitro models of human tissue provide a means to safely, accurately, and rapidly develop therapies for disease. Current in vitro models do not possess appropriate levels of cell function, resulting in an inaccurate representation of in vivo physiology. Mechanical parameters, such as sub-micron topography and flow-induced shear stress (FSS), influence cell functions such as alignment, migration, differentiation and phenotypic expression. Combining, and independently controlling, biomaterial surface topography and FSS in a cell culture device would provide a means to control cell function resulting in more physiologically-representative in vitro models of human tissue. Here we develop the Microscale Tissue Modeling Device (MTMD) which couples a topographically-patterned substrate with a microfluidic chamber to control both topographic and FSS cues to cells. Cells from the human renal proximal tubule cell line HK-2 were cultured in the MTMD and exposed to topographic patterns and several levels of FSS simultaneously. Results show that the biomaterial property of surface topography and FSS work in concert to elicit cell alignment and influence tight junction (TJ) formation, with topography enhancing cell response to FSS. By administering independently-controlled mechanical parameters to cell populations, the MTMD creates a more realistic in vitro model of human renal tissue.

In this article, a PDMS microfluidic immunosensor integrated with specific antibody immobilized alumina nanoporous membrane was developed for rapid detection of foodborne pathogens Escherichia coli O157:H7 and Staphylococcus aureus with electrochemical impedance spectrum. Firstly, antibodies to the targeted bacteria were covalently immobilized on the nanoporous alumina membranes via self assembled (3-glycidoxypropyl)trimethoxysilane (GPMS) silane. Then, the impedance spectrum was recorded for bacteria detection ranging from 1 Hz to 100 kHz. The maximum impedance amplitude change for these two food pathogens was around 100 Hz. This microfluidic immunosensor based on nanoporous membrane impedance spectrum could achieve rapid bacteria detection within 2 h with a high sensitivity of 10² CFU/ml. Cross-bacteria experiments for E. coli O157:H7 and S. aureus were also explored to testify the specificity. The results showed that impedance amplitude at 100 Hz had a significant reduction in binding of bacteria when the membrane was exposed to non-specific bacteria.

We investigate the waveguiding properties of a planar metamaterial slab using terahertz time-domain attenuated total reflection spectroscopy. The enhancement of evanescent waves is observed for transverse electric and transverse magnetic excitation and is caused by resonant excitation of waveguide modes in the slab. Our calculation describes the experimental results and justifies the extremely small effective thickness. We also studied the dispersion relations of the waveguide modes of the slab by theoretical calculation.

We have designed, fabricated, and characterized metamaterial enhanced bimaterial cantilever pixels for far-infrared detection. Local heating due to absorption from split ring resonators (SRRs) incorporated directly onto the cantilever pixels leads to mechanical deflection which is readily detected with visible light. Highly responsive pixels have been fabricated for detection at 95 GHz and 693 GHz, demonstrating the frequency agility of our technique. We have obtained single pixel responsivities as high as 16,500 V/W and noise equivalent powers of 10⁻⁸ W/Hz^1/2 with these first-generation devices.

We have created a low profile, high efficiency half waveplate for operation at terahertz (THz) frequencies. The waveplate is a periodic gold/polyimide composite with a physical thickness of λ/10. Our reflection based waveplate has an intensity throughput of 80% at the design frequency of 350 GHz. This is quite high in comparison to transmissive THz components which typically suffer a large insertion loss due to Fresnel reflections. Simulations suggest a halfwave rotation of over 99% of the reflected THz radiation from 320 – 372 GHz. Experiments at 350 GHz confirm the basic operation of our electromagnetic composite as a functional half waveplate.

Polymeric deformable sensor arrays have been employed to measure cellular forces and offered insights into the study of cellular mechanics. Previous studies have been focused on using transducers in static domain and assumed elastic beam theory as the force conversion model. Neglecting the inherent viscoelastic behavior of polydimethylsiloxane and low aspect ratios of the sensor arrays compromised the accuracy of these devices. In this work, a more in-depth viscoelastic Timoshenko beam model was developed incorporating dynamic cellular forces. We studied chemically stimulated contractions of cardiac myocytes and found that the loading rate has a considerable influence on the sensitivity of the sensor arrays.

A paper-based metamaterial (MM) device, which can potentially be utilized for quantitative analysis in biochemical sensing applications is introduced. Proof-of-concept demonstrations are accomplished by patterning micrometer-sized MM reson­ators on paper substrates and monitoring the resonance shift induced by placing different concentrations of glucose solution on the paper MM.

The application and commercialization of microelectromechanical system (MEMS) devices suffer from reliability problems due to the structural inelastic deformation during device operation. Nanocoatings have been demonstrated to be promising solutions for suppressing creep and stress relaxation in bilayer MEMS devices. However, the micro/nano-mechanics within and/or between microcantilevers and coatings are not fully understood, especially when temperature, time, and geometric and material nonlinearities play significant roles in the thermomechanical responses. In this study, the thermomechanical behavior of alumina-coated/uncoated Au/SiN_x bilayer microcantilevers was characterized by using thermal cycling and isothermal holding tests. Finite element analysis with power-law creep was used to simulate the mechanical behavior of microcantilevers during isothermal holding. To better understand the stress evolution and the mechanism of inelastic deformation, scanning electron microscopy and atomic force microscopy was employed to explore the grain growth and grain boundary grooving after isothermal holding at various temperatures of 100 °C, 150 °C and 200 °C. The methods and results presented in this paper are useful for the fundamental understanding of many similar bilayer microcantilever-based MEMS devices.

In this paper, the mechanical and fracture properties of silicon nitride films subjected to rapid thermal annealing (RTA) have been systemically tested. The residual stress, Young’s modulus, hardness, fracture toughness, and interfacial strength of both sputtered and plasma-enhanced chemical vapor deposition (PECVD) silicon nitride films deposited on silicon wafers were measured and compared. The results indicated that the Young’s modulus and hardness of both types of silicon nitride films significantly increased when the RTA temperature increased. Furthermore, RTA processes could also alter the state of residual stress. The initial residual compressive stress of sputtered silicon nitride film was gradually relieved, and the film became tensile after the RTA process. For PECVD silicon nitride, the tensile residual stress reached its peak after a 600 °C RTA, then dropped after further increases in RTA temperature, due to stress relaxation. The tendency of the equivalent fracture toughness was to exhibit a strong correlation with that shown in the residual stress of silicon nitride. By considering the effect of residual stress, the real fracture toughness of both types of silicon nitride films were slightly enhanced by using RTA processes. Finally, experimental results indicated that the interfacial strength of PECVD silicon nitride could also be significantly improved by RTA processes at 600–800 °C. On the other hand, the initial interfacial strength of the sputtered silicon nitride was sufficiently strong, and the RTA processes only resulted in minor improvements. The characterization flow could be applied to other brittle films, and these specific test results should be useful for improving the structural integrity and process optimization of related MEMS and IC applications.

We present a detailed study of non-planar or ‘stand-up’ split ring resonators operating at terahertz frequencies. Based on a facile multilayer electroplating fabrication, this technique can create large area split ring resonators on both rigid substrates and conformally compliant structures. In agreement with simulation results, the characterization of these metamaterials shows a strong response induced purely by the magnetic field. The retrieved parameters also exhibit negative permeability values over a broad frequency span. The extracted parameters exhibit bianisotropy due to the symmetry breaking of the substrate, and this effect is investigated for both single and broad side coupled split rings. Our 3D metamaterial examples pave the way towards numerous potential applications in the terahertz region of the spectrum.

A thermal conductivity detector (TCD) design that is minimally affected by changing flow rates in the microchannel is presented. In gas chromatography systems, pressure fluctuations can result in false peaks and an unstable baseline, reducing the limit of detection. Theories of TCD operation have suggested that the center region of a detector is minimally affected by changing carrier gas flow rates. However, these theories assume that the detector elements are maintained at a constant temperature along their entire length, which is incorrect. We have performed testing and modeling to determine the accuracy of this assumption and its effects on overall TCD performance. We developed a model of multiple resistively heated TCD elements, which calculated that flow invariance can still be achieved even without perfectly constant temperature elements. A fabricated multiple element TCD was tested to show how the development of a flow rate invariant detector can be used to reduce the complexity and increase the portability of gas chromatography systems.

We present frequency tunable metamaterial designs at terahertz (THz) frequencies using broadside coupled split-ring resonator (BC-SRR) arrays. Frequency tuning, arising from changes in near-field coupling, is obtained by in-plane displacement of the two SRR layers. For electrical excitation, the resonance frequency continuously redshifts as a function of displacement. The maximum frequency shift occurs for vertical displacement of half a unit cell, resulting in a shift of 663 GHz (51% of f₀). We discuss the difference in the BC-SRR response for electrical excitation in comparison to magnetic excitation in terms of hybridization arising from inductive and capacitive coupling.

Microsystems are providing key advances in studying single-cell mechanical behaviours. The mechanical interaction of cells with their extracellular matrix is fundamentally important for cell migration, division, phagocytes and apoptosis. As the displacement and scales of cellular phenomena is comparable to optical wavelength, optical metrology offers superior resolution and real-time imaging capabilities to measure cell forces and subcellular behaviour as compared to its traditional counterparts. This review Letter discusses the principles, formation and methodological aspects of building opto-mechanical systems in studying cell forces. The authors report current advances of various opto-mechanical systems in studying different aspects of cell mechanics.

Recent advances in metamaterials (MMs) research have highlighted the possibility to create novel devices with unique electromagnetic (EM) functionality. Indeed, the power of MMs lies in the fact that it is possible to construct materials with a user-designed EM response at a precisely controlled target frequency. This is especially important for the technologically relevant terahertz (THz) frequency regime with a view toward creating new component technologies to manipulate radiation in this hard to access wavelength range. Considerable progress has been made in the design, fabrication, and characterization of MMs at THz frequencies. This article reviews the latest trends in THz MM research.

Microsystems are providing key advances in studying single cell mechanical behavior. The mechanical interaction of cells with their extracellular matrix is fundamentally important for cell migration, division, phagocytosis and aptoptosis. This review reports the development of microsystems on studying cell forces. Microsystems provide advantages of studying single cells since the scale of cells is on the micron level. The components of microsystems provide culture, loading, guiding, trapping and on chip analysis of cellular mechanical forces. This paper gives overviews on how MEMS are advancing in the field of cell biomechno sensory systems. It presents different materials, and mode of studying cell mechanics. Finally, we comment on the future directions and challenges on the state of art techniques.

Microfabricated polymer transducers have been developed to study cell mechanics. The key principle is to quantify the deformations on the sensor arrays induced by cell contractions and convert them into force distributions. The simplifications in deformation measurements come from the basic assumption that the deformation is solely attributed to cell contractions triggered by chemical or electrical stimuli. The diffraction moiré fringes via two polymer gratings provide whole field evolutions of distortion/strain on soft-lithography fabricated substrates. We found that the moiré patterns are able to decouple predistortions which were traditionally thought to be solely caused by cell contractile forces.

To date, most models of micro-thermal conductivity detectors (µTCDs) positioned in a micro channel have assumed a constant temperature along the entire length of the element. This was known to be an approximation used to make the problem tractable; however, the accuracy of this assumption was unknown. We have designed, fabricated and tested a new µTCD for the purposes of measuring the physical changes in the thermal distribution on a µTCD element with changing flow rate. This device shows, in contrast to the existing models for a thin film µTCD, that due to constantly changing boundary conditions, there is effectively no thermal entry length where the gas obtains thermal equilibrium. Models of µTCD operation cannot use the traditional constant temperature or constant heat flux assumptions and need to include joule heating as well as lateral thermal conduction to accurately simulate device operations.

We present the design, simulation, fabrication and characterization of structurally tunable metamaterials showing a marked tunability of the electric and magnetic responses at terahertz frequencies. Our results demonstrate that structurally tunable metamaterials offer significant potential to realize novel electromagnetic functionality ranging from dynamical filtering to reconfigurable cloaks or detectors. Furthermore, this approach is not limited to terahertz frequencies and may be readily used over much of the electromagnetic spectrum.

We design, fabricate, and characterize split-ring resonator (SRR) based planar terahertz metamaterials (MMs) on ultrathin silicon nitride substrates for biosensing applications. Proof-of-principle demonstration of increased sensitivity in thin substrate SRR-MMs is shown by detection of doped and undoped protein thin films (silk fibroin) of various thicknesses and by monitoring transmission changes using terahertz time-domain spectroscopy. SRR-MMs fabricated on thin film substrates show significantly better performance than identical SRR-MMs fabricated on bulk silicon substrates paving the way for improved biological and chemical sensing applications.

We present the design, fabrication and characterization of a dual band metamaterial absorber which experimentally shows two distinct absorption peaks of 0.85 at 1.4 THz and 0.94 at 3.0 THz. The dual band absorber consists of a dual band electric-field-coupled (ELC) resonator and a metallic ground plane, separated by an 8 µm dielectric spacer. Fine tuning of the two absorption resonances is achieved by individually adjusting each ELC resonator geometry.

Electroactive conducting polymers (CPs) have been frequently used for fabricating bending actuators. To model this type of actuation, the traditional double-layer beam bending theory was implemented by neglecting the thickness of the thin intermediate metal layers for the sake of simplification. However, this common assumption has not been carefully validated and the associated errors have not been well acknowledged. In this work, a generic multilayer bending model was introduced to account for the actuators consisting of an arbitrary number of layers. Our model found the bending curvature, strain, stress, and in particular work density of the multilayer actuator as explicit functions of the thickness and modulus of each individual layer. The thickness of metals and conducting polymers were controlled in thermal evaporation and electrochemical synthesis, respectively. The modulus of polypyrrole (PPy), the conducting polymer used in this work, was determined within our model by the bending curvature measured using the charge-coupled device (CCD). This gave a modulus of our electrochemically synthesized PPy of 80 MPa, corresponding to an actuation strain of 2% in our model. It was concluded that neglecting the intermediate metal layers would lead to substantial errors. For instance, using a PPy/Au/Kapton trilayer actuator, a 5% error or below in strain can only be found if the Au layer is one thousand times thinner than Kapton. To enhance the actuation, a PPy/Pt/PVDF/Pt/PPy five-layer actuator has been often used. In this case, even if the Pt layer was reduced to 10 nm, our predicted error of neglecting the two metal layers would be 12.59%. Our results showed that the work density, chosen to measure the overall performance of the actuator, was highly sensitive to the modulus of the substrate polymer layer so that it was generally desirable of using a soft polymer substrate. With the multilayer bending model, we intend to provide an accurate and reliable tool for systematically analyzing the bending behavior and performance of the CP-based actuators.

Accurate knowledge of flow rate is critical when quantifying analytes in a chromatographic separation. Using the flow rate, the area of a peak and the response factor of a detector one can calculate the total quantity of the analyte being examined. To date, this quantification has not been possible since no in situ method for flow rate detection within a detector existed. We have developed and tested a novel device and method for measuring the linear flow rate in a micro-gas chromatography (GC) system. Our design utilizes a high-sensitivity micro thermal conductivity detector (µTCD), which is capable of replacing a traditional TCD and requires no calibration for the precise measurement of flow rates. Furthermore, this measurement occurs exactly where the solute elutes from the GC separation column, the point at which knowledge of flow rate is most critical for analyte quantification. To the best of our knowledge, no other method of measuring the flow rate directly at the sensor currently exists.

Polydimethylsiloxane (PDMS)-based micropillars (or microcantilevers) have been used as bio-transducers for measuring cellular forces on the order of pN to µN. The measurement accuracy of these sensitive devices depends on appropriate modeling to convert the micropillar deformations into the corresponding reaction forces. The traditional approach to calculating the reaction force is based on the Euler beam theory with consideration of a linear elastic slender beam for the micropillar. However, the low aspect ratio in geometry of PDMS micropillars does not satisfy the slender beam requirement. Consequently, the Timoshenko beam theory, appropriate for a beam with a low aspect ratio, should be used. In addition, the inherently time-dependent behavior in PDMS has to be considered for accurate force conversion. In this paper, the Timoshenko beam theory, along with the consideration of viscoelastic behavior of PDMS, was used to model the mechanical response of micropillars. The viscoelastic behavior of PDMS was characterized by stress relaxation nanoindentation using a circular flat punch. A correction procedure was developed to determine the load–displacement relationship with consideration of ramp loading. The relaxation function was extracted and described by a generalized Maxwell model. The bending of rectangular micropillars was performed by a wedge indenter tip. The viscoelastic Timoshenko beam formula was used to calculate the mechanical response of the micropillar, and the results were compared with measurement data. The calculated reaction forces agreed well with the experimental data at three different loading rates. A parametric study was conducted to evaluate the accuracy of the viscoelastic Timoshenko beam model by comparing the reaction forces calculated from the elastic Euler beam, elastic Timoshenko beam and viscoelastic Euler beam models at various aspect ratios and loading rates. The extension of modeling from the elastic Euler beam theory to the viscoelastic Timoshenko beam theory has improved the accuracy for the conversion of the PDMS micropillar deformations to forces, which will benefit the polymer-based micro bio-transducer applications.

Fabrication and characterization of the first large area metamaterial structures patterned on free-standing biocompatible silk substrates is reported. Strong resonance responses at terahertz frequencies are shown, providing a promising path towards the development a new class of metamaterial-inspired bioelectric and biophotonic devices.

The mapping of contraction forces developed from cells to their extracellular matrix is crucial to understanding how cells regulate their physiological function to adapt to their living environment and cellular processes. This paper reports a novel cell contractility mapping transducer utilizing moiré patterns as a visual and quantitative tool. Coherent light diffracted from two closely placed microfabricated periodic substrates is capable of mapping cell contraction forces via mapping the in-plane displacement on the sample substrate. By integrating cell culture environment and automated Fourier-based fringe analysis, the moiré pattern generated through microfabricated periodic substrates enables the mapping of cell contraction force distribution through phase changes encoded in carrier moiré fringe patterns. We demonstrated utilizing the transducer to map cardiac myocyte contraction under electric stimulation and vascular smooth muscle cell contractility evolutions triggered by agonist. Given the unique properties of optical moiré techniques (i.e., their automatic displacement and strain contouring and their magnification effect for small displacements), this new approach would be an improvement over existing techniques since it can be integrated with the existing engineered substrates and provide a direct contour of cell forces and fast detection of abnormal cell contractions.

Abnormal vascular cell contractile performance is a hallmark of cardiovascular diseases. Conventional cell force measurement technique requires individually tracking the sensing units and complex computation efforts for further studying cell contractility. We developed instead a robust and simple compact optical moiré system that measures phase changes encoded in carrier moiré patterns generated from two layers of gratings. Cell mechanics study including cell contractile forces and stress and strain distributions during normal and abnormal cell contractions can thus be conveniently analyzed. The distinct signals from moiré patterns in longitudinal and transverse directions revealed abnormal cell mechanical contractility linked to cardiovascular disease.

The monolithic integration of a high sensitivity detector with a gas chromatography (GC) separation column creates many potential advantages over the discrete components of a traditional chromatography system. In miniaturized high-speed GC systems, component interconnections can cause crucial errors and loss of fidelity during detection and analysis. A monolithically integrated device would eliminate the need to create helium-tight interconnections, which are bulky and labor intensive. Additionally, batch fabrication of integrated devices that no longer require expensive and fragile detectors can decrease the cost of micro GC systems through economies of scale. We present the design, fabrication and operation of a monolithic GC separation column and detector. Our device is able to separate nitrogen, methane and carbon dioxide within 30 s. This method of device integration could be applied to the existing wealth of column geometries and chemistries designed for specialized applications.

In this article, we introduce and provide details on a large-scale, cost-effective pathway to fabricating ultrahigh dense CuO nanowire arrays by thermal oxidation of Cu substrates in oxygen ambient. The CuO nanowires that are produced at ~500 °C for ~150 min feature an average length and diameter of ~15 µm and ~200 nm, respectively. The room temperature device-related characteristics such as transport, analyte detection and opto-electronic response of individual CuO nanowires have been probed by fabricating single CuO nanowire devices with the use of lift-off photolithographical techniques. The experiments confirm that as-grown nanowires are of p-type, have a band gap of ~1.4 eV, and show strong sensitivity to both NO₂ and NH₃ gases. The devices also showed strong response to white light with device responsivity approaching ~8 A/W for optical power densities of only ~1 mW/cm². Additionally, a complex interaction of photoproduced electron−hole pairs with the surface-originating chemisorbed agents including O₂ and NO₂ is found to drastically affect the gas sensitivity of CuO nanowire-based devices, where photoinduced adsorption of the analyte enhances the device response.

We demonstrate reconfigurable anisotropic metamaterials at terahertz frequencies where artificial “atoms” reorient within unit cells in response to an external stimulus. This is accomplished by fabricating planar arrays of split ring resonators on bimaterial cantilevers designed to bend out of plane in response to a thermal stimulus. We observe a marked tunability of the electric and magnetic response as the split ring resonators reorient within their unit cells. Our results demonstrate that adaptive metamaterials offer significant potential to realize novel electromagnetic functionality ranging from thermal detection to reconfigurable cloaks or absorbers.

Polydimethylsiloxane (PDMS) is an important polymeric material widely used in bio-MEMS devices such as micropillar arrays for cellular mechanical force measurements. The accuracy of such a measurement relies on choosing an appropriate material constitutive model for converting the measured structural deformations into corresponding reaction forces. However, although PDMS is a well-known viscoelastic material, many researchers in the past have treated it as a linear elastic material, which could result in errors of cellular traction force interpretation. In this paper, the mechanical properties of PDMS were characterized by using uniaxial compression, dynamic mechanical analysis, and nanoindentation tests, as well as finite element analysis (FEA). A generalized Maxwell model with the use of two exponential terms was used to emulate the mechanical behavior of PDMS at room temperature. After we found the viscoelastic constitutive law of PDMS, we used it to develop a more accurate model for converting deflection data to cellular traction forces. Moreover, in situ cellular traction force evolutions of cardiac myocytes were demonstrated by using this new conversion model. The results presented by this paper are believed to be useful for biologists who are interpreting similar physiological processes.

Cardiomyocyte death caused by proinflammatory cytokines, such as Tumor necrosis factor α (TNF-α), is one of the hot topics in cardiovascular research. TNF-α can induce multiple cell processes that are dependent on the treatment time although the long-term treatment definitely leads to cell death. The ability to intervene in cell death will be invaluable to reveal the effects of short-term TNF-α treatment to cardiomyocytes. However, a real-time monitoring technique is needed to guide the intervention of cell responses. In this work, we employed the impedance-sensing technique to real-time monitor the equivalent cell–substrate distance of cardiomyocytes via electrochemical impedance spectroscopy (EIS) and electrical cell–substrate impedance sensing (ECIS). In the stabilized cardiomyocyte culture, the sustained TNF-α treatment caused strengthened cell adhesion in the first 2 h which was followed by the transition to cell detachment afterwards. Considering cell detachment was an early morphological evidence of cell death, we removed TNF-α from the cardiomyocyte culture before the transition to achieve the intervention of cell responses. The result of this intervention showed that cell adhesion was continuously strengthened before and after the removal of TNF-α, indicating the short-term treated cardiomyocytes did not undergo death processes. It was also demonstrated in TUNEL and TBE tests that the percentages of apoptosis and cell death were both lowered.

Bimaterial microcantilevers are used in numerous applications in microelectromechanical systems (MEMS) for thermal, mechanical, optical, tribological and biological functionalities. Unfortunately, the residual stress-induced curvature and combined effects of creep and stress relaxation in the thin film significantly compromises the performance of these structures. To fully understand the themomechanical deformation and microstructural evolution of such microcantilevers, SiN_x/Al bilayer cantilever beams were studied in this work. These microcantilevers were heated and subsequently cooled for five cycles between room temperature and 250 °C, with the peak temperature in each successive cycle increased in increments of 25 °C using a custom-built micro-heating stage. The in situ curvature change was monitored using an interferometric microscope. The general behavior of the bimaterial microcantilever beams can be characterized by linear thermoelastic regimes with (dκ/dT)_ave = 0.079 mm⁻¹ °C⁻¹ and inelastic regimes. After thermal cycling with a maximum temperature of 225 °C, upon returning to room temperature, the bimaterial microcantilever beams were flattened and the curvature decreased by 99%. The thermoelastic deformation during thermal cycling was well described by the Kirchhoff plate theory. Deformation of bimaterial microcantilevers during long-term isothermal holding was studied at temperatures of 100 °C, 125 °C and 150 °C with a holding period of 70 h. The curvature of bimaterial microcantilever beams decreased more for higher holding temperatures. Finite element analysis (FEA) with power-law creep in Al was used to simulate the creep and stress relaxation and thus the curvature change of the bimaterial microcantilever beams. The microstructure evolutions due to isothermal holding in SiN_x/Al microcantilevers were studied using an atomic force microscope (AFM). The grain growth in both the vertical and lateral directions was present due to isothermal holding. As the isothermal holding temperature increased, the surface roughness of the film increased with more prominent grain structures.

Deregulated cardiomyocyte death is a critical risk factor in a variety of cardiovascular diseases. Although various assays have been developed to detect cell responses during cell death, the capability of monitoring cell detachment will enhance the understanding of death processes by providing instant information at its early phase. In this work, we developed an impedance-sensing assay for real-time monitoring of cardiomyocyte death induced by tumor necrosis factor-α based on recording the change in cardiomyocyte adhesion to extracellular matrix. Electrochemical impedance spectroscopy was employed in impedance data processing, followed by calibration with the electrical cell-substrate impedance-sensing technique. The adhesion profile of cardiomyocytes undergoing cell death processes was recorded as the time course of equivalent cell-substrate distance. The cell detachment was detected with our assay and proved related to cell death in the following experiments, indicating its advantage against the conventional assays, such as Trypan blue exclusion. An optimal concentration of tumor necrosis factor-α (20 ng/mL) was determined to induce cardiomyocyte apoptosis rather than the combinative cell death of necrosis and apoptosis by comparing the concentration-related adhesion profiles. The cardiomyocytes undergoing apoptosis experienced an increase of cell-substrate distance from 59.1 to 89.2 nm within 24 h. The early change of cell adhesion was proved related to cardiomyocyte apoptosis in the following TUNEL test at t = 24 h, which suggested the possibility of early and noninvasive detection of cardiomyocyte apoptosis.

Abnormal vascular smooth muscle cell (VSMC) contraction plays an important role in vascular diseases. The RhoA/ROCK signaling pathway is now well recognized to mediate vascular smooth muscle contraction in response to vasoconstrictors by inhibiting myosin phosphatase (MLCP) activity and increasing myosin light chain phosphorylation. Two ROCK isoforms, ROCK1 and ROCK2, are expressed in many tissues, yet the isoform-specific roles of ROCK1 and ROCK2 in vascular smooth muscle and the mechanism of ROCK-mediated regulation of MLCP are not well understood. In this study, ROCK2, but not ROCK1, bound directly to the myosin binding subunit of MLCP, yet both ROCK isoforms regulated MLCP and myosin light chain phosphorylation. Despite that both ROCK1 and ROCK2 regulated MLCP, the ROCK isoforms had distinct and opposing effects on VSMC morphology and ROCK2, but not ROCK1, had a predominant role in VSMC contractility. These data support that although the ROCK isoforms both regulate MLCP and myosin light chain phosphorylation through different mechanisms, they have distinct roles in VSMC function.

We have fabricated a quarter-wave plate from a single layer of birefringent electric split-ring resonators (ELC). For comparison, an appropriately scaled double layer meanderline structure was fabricated. At the design frequency of 639 GHz, the ELC structure achieves 99.9% circular polarization while the meanderline achieves 99.6%. The meanderline displays a larger bandwidth of operation, attaining over 99% circular polarization from 615 – 743 GHz, while the ELC achieves 99% from 626 – 660 GHz. However, both are broad enough for use with CW sources making ELCs a more attractive choice due to the ease of fabrication. Both samples are free standing with a total thickness of 70 µm for the meanderline structure and a mere 20 µm for the ELC highlighting the large degree of birefringence exhibited with metamaterial structures.

Polydimethylsiloxane (PDMS) microcantilevers have been used as force sensors for studying cellular mechanics by converting their displacements to cellular mechanical forces. However, PDMS is an inherently viscoelastic material and its elastic modulus changes with loading rates and elapsed time. Therefore, the traditional approach to calculating cellular mechanical forces based on elastic mechanics can result in errors. This letter reports a more in-depth method for viscoelastic characterization, modeling, and analysis associated with the bending behavior of the PDMS microcantilevers. A viscoelastic force conversion model was developed and validated by proof-of-principle bending tests.

We present the design, fabrication, and characterization of a metamaterial absorber which is resonant at terahertz frequencies. We experimentally demonstrate an absorptivity of 0.97 at 1.6 THz. Importantly, our absorber is only 16 µ⁢m thick, resulting in a highly flexible material that, further, operates over a wide range of angles of incidence for both transverse electric and transverse magnetic radiation.

We have fabricated resonant terahertz metamaterials on free-standing polyimide substrates. The low-loss polyimide substrates can be as thin as 5.5 µm yielding robust large-area metamaterials which are easily wrapped into cylinders with a radius of a few millimeters. Our results provide a path forward for creating multi-layer non-planar metamaterials at terahertz frequencies.

We present novel metamaterial structures based upon various planar wallpaper groups, in both hexagonal and square unit cells. An investigation of metamaterials consisting of one, two, and three unique sub-lattices with resonant frequencies in the terahertz (THz) was performed. We describe the theory, perform simulations, and conduct experiments to characterize these multiple element metamaterials. A method for using these new structures as a means for bio/chemical hazard detection, as well as electromagnetic signature control is proposed.

Cells alter their shape and morphology and interact with their surrounding environment. Mechanical forces developed by cells to their surrounding environments are fundamental to many physiological processes, such as cell growth, division, migration and apoptosis. In this paper, a novel optical Moiré based biomechanol force sensor was developed for cell traction force mapping. We utilized coherent laser beams to illuminate periodic polymeric substrates where isolated cells were cultured. We demonstrated one-dimensional and two-dimensional traction force mapping via optical Moiré for both cardiac myocytes and vascular smooth muscle cells. The magnification effect of the Moiré fringe pattern permits a real time monitoring of the mechanical interaction between isolated cells and their underlying periodic polymeric structures.

This letter reports an approach for cell contraction force mapping by utilizing optical moiré effect. We cultured living cells on patterned polymer substrates and studied the diffraction moiré patterns. We found that the flexible moiré patterns generated on the periodic substrates are capable of mapping cell contraction force evolution in whole field. We demonstrated one- and two-dimensional force mappings in vascular cells. Due to moiré magnification, this imaging approach can provide a versatile visual tool for mapping the cell-substrate interactions in living cells.

Silicon oxynitride (SiON) has received a great deal of attention in micro-electro-mechanical system (MEMS) integration due to its composition-dependent tunability in optical, electronic and mechanical properties. In this work, silicon oxynitride films with different oxygen and nitrogen content were deposited by RF magnetron sputtering. Energy dispersive X-ray (EDX) spectroscopy and Fourier-transform infrared (FT-IR) spectroscopy were employed to characterize the SiON films with respect to stoichiometric composition and atomic bonding structure. Time-dependent plastic deformation (creep) of SiON films were investigated by depth-sensing nanoindentation at room temperature. Young’s modulus and indentation-hardness were found correlated with the nitrogen/oxygen ratio in SiON films. Results from nanoindentation creep indicated that plastic flow was less homogenous with increasing nitrogen content in film composition. Correspondingly, a deformation mechanism based on atomic bonding structure and shear transformation zone (STZ) plasticity theory was proposed to interpret creep behaviors of sputtered SiON films.

This paper reports the recent progress on the development of double-cantilever infrared (IR) detectors, including the fabrication, the post-process curvature control, and also the first-time demonstration of thermal detection using capacitive-based IR focal plane arrays (FPAs). In this work, simplified double-cantilever IR FPAs based on bimaterial SiN_x/Al and Al/SiNx cantilevers are fabricated using a surface micromachining module with polyimide as the sacrificial material. Thermal-cycling experiments of both 200 nm-thick Ebeam Al and 200 nm-thick PECVD SiN_x films reveal that the residual stresses in IR materials can be significantly modified by thermal annealing. Therefore, an engineering approach to flattening IR FPAs is developed by using rapid thermal annealing (RTA). This article also demonstrates the thermal detection of cantilever IR FPAs using commercialized weak capacitance readout IC.

Uncooled microcantilever-based infrared (IR) detectors have recently gained interest due to their low noise equivalent temperature difference (NETD), while concurrently maintaining low costs. These properties have made them available for a wider range of applications. However, the curvature induced by residual strain mismatch severely compromises the device’s performance. Therefore, to meet performance and reliability requirements, it is important to fully understand the deformation of IR detectors. In this study, bimaterial (SiN_x/Al) microcantilever-based IR detectors were fabricated using surface micromachining with polyimide as a sacrificial layer. Thermo-mechanical deformation mechanisms were studied through the use of thermal cycling. A temperature chamber with accurate temperature control and an interferometer microscope were adopted in this study for thermal cycling and full-field curvature measurements. It was found that thermal cycling reduced the residual strain mismatch within the bimaterial structure and thus flattened the microcantilever-based IR detectors. Specifically, thermal cycling with a maximum temperature of 295 °C resulted in a 97% decrease in curvature of the microcantilever-based IR detectors upon return to room temperature. The thermoelastic deformation of the IR detectors was modeled using both finite element method (FEM) and analytical methods. A modified analytical solution based on plate theory was established to describe the thermoelastic mechanical responses by using a correction factor derived from FEM. Although in the current study Al and SiN_x were chosen for the application of microcantilever-based IR detectors, the general experimental protocol and modeling approach can be applied to describe thermoelastic mechanical responses of bimaterial devices with different materials. Toward the end of this paper, we studied the correction factors in the modified analytical solution while varying parameters such as Young’s modulus ratio, thickness ratio and coefficient of thermal expansion (CTE) mismatch to investigate the influences of these parameters.

We present a metamaterial that acts as a strongly resonant absorber at terahertz frequencies. Our design consists of a bilayer unit cell which allows for maximization of the absorption through independent tuning of the electrical permittivity and magnetic permeability. An experimental absorptivity of 70% at 1.3 terahertz is demonstrated. We utilize only a single unit cell in the propagation direction, thus achieving an absorption coefficient α=2000 cm^-1. These metamaterials are promising candidates as absorbing elements for thermally based THz imaging, due to their relatively low volume, low density, and narrow band response.

Plasma-enhanced chemical vapor deposited (PECVD) silicon oxide (SiO_x) thin films have been widely used in Micro/Nano Electro Mechanical Systems to form electrical and mechanical components. In this paper, we explore the use of nanoindentation techniques as a method of measuring equivalent stress–strain curves of the PECVD SiO_x thin films. Four indenter tips with different geometries were adopted in our experiments, enabling us to probe the elastic, elasto-plastic, and fully plastic deformation regimes of the PECVD SiO_x thin films. The initial yielding point (σ_I) and stationary yielding point (σ_II) are separately identified for the as-deposited and annealed PECVD SiO_x thin films, as well as a standard fused quartz sample. Based on the experimental results, a shear transformation zone based amorphous plasticity theory is applied to depict the plastic deformation mechanism in the PECVD SiO_x.

The cell−substrate distance is a direct indicator of cell adhesion to extracellular matrix which is indispensable in cell culture. A real-time monitoring approach can provide a detailed profile of cell adhesion, so that enables the detecting of adhesion-related cell behavior. In this work, we report a novel real-time impedance-based method to record the adhesion profile of cardiomyocyte, overcoming its inscrutability due to the primary culture. Microfabricated biosensors are applied in cardiomyocyte culture after characterizing the cell-free system. Cyclic frequency scanning data of cell-related impedance are generated and automatically fit into the equivalent circuit model, which is established using electrochemical impedance spectroscopy. The data are displayed as the alteration of normalized cell−substrate distance and the essential parameters for manual electric cell−substrate impedance sensing calibration of absolute distance. The time course displays a significant decline in the equivalent cell−substrate distance, from 155.8 to 60.2 nm in the first 20 h of cardiomyocyte culture. Furthermore, the cardiomyocytes cultured in long-term medium and short-term medium (ACCT) for 10 h exhibit distinct difference in adhesion rate as well as cell−substrate distance (72 vs 68 nm).

Nanostructures with controllable geometries are increasingly needed for mechanosensor applications due to their superior capabilities in measuring minute mechanical forces at small scales. Previous nanofabrication technologies are deficient in creating such structures due to high cost, complicated process and poor geometric control. In this paper, we introduce an innovative use of fluorine enhanced oxidation to realize a segment by segment sidewall control strategy for creating silicon nanostructures. This method is based on fluorine reactive ion etching, where the rich amount of fluorine species accumulated on the surface are utilized to enhance surface oxidation during an additional oxygen exposure. The experimental results showed good repeatability. The characteristic feature size of the resulting nanostructures is about 200 nm. The minimum feature size goes down to 50 nm. This method requires minimal use of equipment, and demonstrates good control of sidewall profiles. It thus has technical and economic significance in development of functional nanomechanosensors, which are potent for quantitative mechanical sensing and actuating at small scales.

A number of techniques have been developed to monitor contractile function in isolated cardiac myocytes. While invaluable observations have been gained from these methodologies in understanding the contractile processes of the heart, they are invariably limited by their in vitro conditions. The present challenge is to develop innovative assays to mimic the in vivo milieu so as to allow a more physiological assessment of cardiac myocyte contractile forces. Here we demonstrate the use of a silicone elastomer, poly(dimethylsiloxane) (PDMS), to simultaneously orient adult cardiac myocytes in primary culture and measure the cellular forces in a three-dimensional substrate. The realignment of adult cardiac myocytes in long-term culture (7 days) was achieved due to directional reassembly of the myofibrils along the parallel polymeric sidewalls. The cellular mechanical forces were recorded in situ by observing the deformation of the micropillars embedded in the substrate. By coupling the cellular mechanical force measurements with on-chip cell orientation, this novel assay is expected to provide a means of a more physiological assessment of single cardiac myocyte contractile function and may facilitate the future development of in vitro assembled functional cardiac tissue.

Bimaterial SiN_x/Al infrared cantilever structures are always initially curved because of the imbalanced residual stress in the two layers. Their performance and functionality are therefore significantly decreased. A thorough study of the residual stress (strain) has then become a key issue in the development of bimaterial SiN_x/Al cantilever structures. In the curvature-based approach to the film stress, the residual strain is derived from the measured curvature based on certain assumptions on the distribution of the residual strain in the thickness direction. Previous models for a bimaterial cantilever structure, however, are not sufficient to characterize the residual strain in bimaterial SiN_x/Al infrared structures. The main goal of this paper is to investigate gradient residual strain in bimaterial SiN_x/Al infrared structures. To achieve this goal, the relationship between the residual strain and bending curvature is developed with the assumption that the residual strain in each layer is linearly distributed rather than uniform throughout the thickness. The profile of the gradient strain is then derived from the curvatures measured during the continuous etching of the top-most SiN_x in the bimaterial cantilevers. The derived residual strain can then be inverted to predict curvature change further in the etching process. This paper demonstrates that a linear assumption of the residual strain yields a stronger agreement with the measured data in comparison to previously used models. In addition, several factors that may affect measurement accuracy are discussed at the end of the paper.

Myofibrils are functional contractile elements in skeleton and cardiac muscle cells of vertebrates and in skeleton muscle cells of invertebrates. Each myofibril consists of parallel filaments, allowing maximal contraction performance. When these muscle cells are cultured in vitro, this parallel arrangement is often disrupted due to cell remodeling. The contractile performance of the muscle cells is thus deteriorated. In this paper, we present a microstructured polymeric substrate to regulate the reassembly of the myofibrils during cell remodeling. The results show the myofibrils in long-term cultured heart muscle cells are reassembled directionally, along longitudinal axis of the microstructured polymeric sidewalls. This directional arrangement is further validated by monitoring the mechanical performance of the living cells, using embedded polymeric microstructures. This work opens a door for orientation of subcellular elements using microstructured extracellular environments, and is the basis for development of in vitro assembled muscle patches.

Cardiac hypertrophy is the heart’s response to a variety of extrinsic and intrinsic stimuli that impose increased biomechanical stress that may be regulated by growth factor such as Endothelin-1 (ET-1). The majority of existing techniques to monitor hypertrophy in vitro are based on florescence probes designed to show morphological and biochemical alterations indicative of cardiomyocyte hypertrophy. In this work, a new cardiomyocyte-based impedance sensing system with the assistance of dielectrophoresis (DEP) cell concentration is developed to monitor the dynamics process of ET-1 induced cardiomyocyte hypertrophy. This device can increase the sensitivity of the impedance system and also has the potential to reduce the time for detection by a significant factor.

Cardiac tissue engineering has evolved as a potential therapeutic approach to assist cardiac regeneration. Controlling the preferential cell orientation of engineered heart tissues is a key issue in cardiac tissue engineering. Here, we present a novel method to construct a model-engineered cardiac tissue-like structure with anisotropic properties. Our analysis shows that the electro-torque which acts on a cylindrical or rod shape cell is zero whenever the electric field is aligned with one of its principal axes. With the interdigitated–castellated microelectrodes, the induction of dielectrophoresis and electro-orientation can accumulate cells and form a tissue-like structure with orientation along the ac electric field. Both experiments and analysis indicate that a large orientation torque and force can be achieved with appropriate frequency and low conductive medium. Finally, we report basic structural and biophysical anisotropy of electro-oriented structure through electromechanical experiments.

Cardiac hypertrophy is an established and independent risk factor for the development of heart failure and sudden cardiac death. At the level of individual cardiac myocytes (heart muscle cells), the cell morphology alters (increase in cell size and myofibrillar re-organization) and protein synthesis is activated. In this paper, a novel cardiomyocyte-based impedance sensing system with the assistance of dielectrophoresis cell concentration is reported to monitor the dynamic process of endothelin-1-induced cardiomyocyte hypertrophy. A dielectrophoresis (DEP) microfluidic device is fabricated capable of concentrating cells from a dilute sample to form a confluent cell monolayer on the surface of microelectrodes. This device can increase the sensitivity of the impedance system and also has the potential to reduce the time for detection by a significant factor. To examine the feasibility of this impedance sensing system, cardiomyocytes are treated with endothelin-1 (ET-1), a known hypertrophic agent. ET-1 induces a continuous rise in cardiomyocyte impedance, which we interpret as strengthening of cellular attachments to the surface substrate. An equivalent circuit model is introduced to fit the impedance spectrum to fully understand the impedance sensing system.

Multilayered structures are widely used as sensing or actuating components in MEMS devices. Since the thin films of multilayered structures are always subject to residual stresses, it is important to model the relation between these residual stresses and the resultant elastic deformation. The main purpose of this paper is to explore two different approaches to addressing this issue when the residual stress in each thin film is not necessarily uniform throughout the thickness. These two approaches are first briefly introduced and then used to arrive at identical solutions for a monolayer cantilever and a bilayer cantilever, both with arbitrary residual strain distributions throughout the thickness. The analytical formulas for a bilayer cantilever are further verified by the numerical simulation of a special case. After the discussion on the errors induced by assuming the gradient residual strains in the bilayer cantilevers are uniform, the relation between the bending plane and the neutral plane in bilayer cantilevers is also explored. Finally, we present an approach to characterizing residual stresses in thin films by using micromachined bilayer cantilevers in conjunction with the theory developed in this paper.

The time-dependent plastic properties of both as-deposited and annealed plasma-enhanced chemical vapor deposited (PECVD) silicon oxide (SiO_x) thin films were probed by nanoindentation creep tests at room temperature. Our experiments found a strong size effect in the creep responses of the as-deposited PECVD SiO_x thin films, which was much reduced after annealing. Based on the experimental results, the deformation mechanism is depicted by the ‘shear transformation zone’ (STZ) based amorphous plasticity theories.

Recently, the ability to create engineered heart tissues with a preferential cell orientation has gained much interest. Here, we present a novel method to construct a cardiac myocyte tissue-like structure using a combination of dielectrophoresis and electro-orientation via a microfluidic chip. The device includes a top home-made silicone chamber containing microfluidic channels and bottom integrated microelectrodes which are patterned on a glass slide to generate dielectrophoresis force and orientation torque. Using the interdigitated-castellated microelectrodes, the induction of a mutually attractive dielectrophoretic force between cardiac myocytes can lead to cells moving close to each other and forming a tissue-like structure with orientation along the alternating current (ac) electric field between the microelectrode gaps. Both experiments and analysis indicate that a large orientation torque and force can be achieved by choosing an optimal frequency around 2 MHz and decreasing the conductivity of medium to a relatively low level. Finally, electromechanical experiments and biopolar impedance measurements were performed to demonstrate the structural and functional anisotropy of electro-oriented structure.

The time-dependent plastic deformation (creep) behaviours of both as-deposited and annealed plasma-enhanced chemical vapour deposited (PECVD) silicon oxide (SiO_x) films were probed by nanoindentation load relaxation tests at room temperature. Our experiments found a strong size effect in the creep responses of the as-deposited PECVD SiO_x thin films, which was much reduced after rapid thermal annealing. Based on the experimental results, the deformation mechanism is depicted by the ‘shear transformation zone’ (STZ)-based amorphous plasticity theories. The physical origin of the STZ is elucidated and linked with the shear banding dynamics. It is postulated that the high strain gradient at shallow indentation depths may be responsible for the reduction in the stress exponent n = ∂ log(strain rate)/∂ log(stress), characteristic of a more homogeneous flow behaviour.

The nanometer-scale in-plane deformation of a PDMS nanostructure array was demonstrated using a geometric moiré technique. The polymer nanostructures with cylindrical profile were fabricated by using the combination of e-beam lithography, reactive ion etching, and replica molding. The moiré pattern is generated by the inference between the polymer nanostructures and the scanning lines of a CCD video camera. A uniform thermal expansion was induced in the polymer nanostructures. The moiré pattern change was observed at different temperatures from 298 to 398 K. The change of the strain/pitch in the nanostructures as the temperature varied was calculated from the pure extension and the angular moiré fringes. The results show the feasibility of using such polymer nanostructures as force sensors to measure the mechanical forces on the order of a few nanonewtons or less. This work has a representative application in mechanics study of the biological objects, e.g. living cells and protein, under their environmental conditions.

This paper reports an approach to eliminating stress-induced curvature in microcantilever-based infrared focal plane arrays (FPAs). Using a combination of argon ion beam machining and rapid thermal annealing (RTA), we successfully modified curvatures of free-standing SiN_x/Al bimaterial FPAs. The SiN_x/Al FPAs were fabricated using a surface micromachining technique with polyimide as a sacrificial material. The as-fabricated FPAs were concavely curved because of the imbalanced residual stresses in the two materials. To modify the FPAs curvature, first, Ar ions with energies of 500 eV was used, which sputter etched PECVD SiN_x at a rate of 4 nm/min, and 20 min of ion beam machining reduced the FPAs curvature from –1.92 to –0.96 mm⁻¹. Then based on the investigation on the thermomechanical behavior of both the e-beam Al and PECVD SiN_x films during the thermal cycling, RTA was proposed to further modify the FPAs curvature. It is found that 5 min of RTA at 375 °C resulted in flat FPAs with acceptable curvatures (<0.10 mm⁻¹).

This paper reports a PDMS microchip for subcellular mechanics study, which consists of micrometer scale polymeric pillars on a plain substrate for cellular force measurement. The chip reported here differs from those proposed in previous work in that a flexible polymer microfabrication technique was applied for manufacturing the polymeric microstructures with various aspect ratios. This allows for the measurement of a range of forces encountered at the subcellular level. The microchip was integrated with a perfusion chamber to allow for precise control of the extracellular environment, thus facilitating cellular mechanics studies under various physiological conditions. A biocompatibility test was carried out using two types of mammalian cells (fibroblasts and cardiac myocytes). The results show satisfactory abundances of both types of cells, with no adverse affect of the PDMS microchip on cell viability. A proof-of-concept force measurement was performed in single contracting cardiac myocytes. The force distribution with a subcellular resolution conforms to the physiologic behavior of cardiac myoctes, indicating the potential application of the reported microchip in subcellular mechanics study.

This paper introduces a novel 3D manufacturing approach to the rapid processing of microfluidic components such as embedded channels and microvalves, using a scanning laser system. Compared to existing manufacturing techniques, our direct UV laser writing method greatly simplifies fabrication processes, potentially reducing the design-to-fabrication time to a few hours, which is extremely beneficial during the product development stages. The initial process validation has been presented by using SU-8 material. With the fine-tuning of the laser processing parameters, the depth of SU-8 polymerization can be controlled. This paper also describes the underlying theory and method to determine the Young’s modulus of the exposed SU-8 material by using a laser acoustic microscopy system. The laser-based ultrasonic technique offers a non-contact, nondestructive means of evaluation and materials characterization. More importantly, it allows for local inspection of material properties. The results presented in this paper potentially could serve as the first crucial step towards the rapid manufacturing of microdevices for lab-on-chip applications.

Plasma-enhanced chemical vapor deposited (PECVD) silane-based silicon oxide (SiOx) films were chosen as an example to study the thermally-induced stress relaxation phenomena of amorphous dielectric films. Wafer curvature was measured optically both during and after various thermal conditions, including temperature cycling, constant peak temperature annealing, and varying peak temperature annealing experiments. From these measurements, we are able to obtain a complete stress evolutional history in the thin film, and further derive a series of mechanical/material properties such as viscoelasticity, density and viscosity, coefficient of thermal expansion (CTE), etc. We found that the stress in the amorphous silicon oxide films is sensitive to both viscosity and density changes. Stress relaxation was then theoretically investigated by a viscoelastic model, which was associated with a defects-based microstructural causal mechanism. Our theoretical model elucidates defects movement and consequent microstructure rearrangement as a source of damping, accompanied by viscous flow. This theory was applied to explain a series of experimental results, including stress hysteresis generation and reduction, stress relaxation, and CTE changes, etc.

This paper reports a flexible fabrication process to manufacture polymeric microstructures with various aspect ratios using a micro molding process. In this study, a silicon template with deep holes (2 µm in diameter) was fabricated and a two-phase vacuum pressure-assisted process was conducted. The liquid polymer contacting with the template was raised into the deep holes due to the differential pressure between the trapped air in the holes and the ambient pressure. The microstructures were solidified by thermal curing at an elevated temperature (65 °C). The height of the resulting polymeric microstructures, ranging from submicron to more than 10 µm, was controlled by tuning the operation parameters in the micro molding process. A proof-of-principle experiment was carried out in isolated cardiac myocytes to measure the cellular forces towards the polymeric microstructures. Moreover, simultaneous cell alignment and force measurement was demonstrated using an advanced structure fabricated using such a technique. This approach reported here differs from the existing ones in that it enables the "1-to-n" replication, thereby lowering the fabrication cost and complexity. This study has a potential application in a variety of polymer-based micro total analysis systems, especially where the polymer microstructures serve as mechanical sensors.

Using the Stoney formula and its modifications, curvature-based techniques are gaining increasingly widespread application in evaluating the stress in a film on a substrate. In principle, the formula applies only when the stress is uniform throughout the film thickness. The main purpose of this paper is to extend the Stoney formula when the residual strain in the film is no longer uniform, but dependent on the z position. To achieve this goal, a general theory was introduced for the elastic deformation of an arbitrary, multilayered system. By practicing this general theory, we used a polynomial function to describe the gradient stress in a film, and contributions by different elements of the polynomial to both the curvature and the bending strain were derived. A finite element simulation for a typical film–substrate structure was then carried out, leading to the verification of the theory developed in this paper. In the discussion section, we explored the relation between the surface curvature and the bending curvature as well as the difference between the stress in the constrained planar state and that in the relaxed state. In addition, the accuracy of the simplified formula, using thin film approximation, was evaluated. Finally, a SiN_x-Al MEMS structure was studied by using the formula in this paper.

In this paper, we developed a scanning laser system, which allows rapid processing of freeform multi-layered microstructures. More importantly it enables rapid prototyping of three-dimensional (3D) microdevices at low cost. The capabilities of three-dimensional manufacturing, inclined patterning, and multi-layered manufacturing have been demonstrated. Specifically, both in-plane and out-of-plane processing is feasible using spot-by-spot controllable laser pulsing. The laser processing perpendicular to the specimen surface is realized by fine tuning the focus level and laser intensity. A large number of microfluidic components such as cantilever beams, embedded channels and other shapes requiring gaps between layers are demonstrated in a single layer. Compared to the existing manufacturing techniques, our direct laser writing method greatly simplifies fabrication processes, potentially reducing the design-to-fabrication cycle to a few hours.

This paper presents cellular mechanics study in isolated cardiac myocytes using a micro-molded polydimethylsiloxane (PDMS) pillars array. The PDMS pillars were fabricated using a specialized micro-molding process where template fabrication and polymer replication were minimized. The spring constant of the subject pillars was determined considering the enlarged root and scallop-like notches. The cellular mechanical force was derived from displacements of individual pillars upon multiplication with the locally determined spring constant. Experiments were conducted to achieve the subcellular force distribution within single cardiac myocyte. Furthermore, the force response of isolated cardiac myocytes upon an isoproterenol perfusion was in situ monitored. The measurements conform to the effect of applied chemical stimulus, suggesting that this approach has the physiology and pathophysiology potential to be a basis for subcellular mechanics study in cardiomyopathy field.

Polymeric material has been utilized as mechanical sensors to measure microscopic cellular forces. Since many polymers are not readily compatible with conventional lithography, fabrication of numerous molds is inevitably a part of the process, compromising low cost and process simplicity. In this letter, we apply a flexible fabrication process to manufacture polymeric mechanical sensors with various aspect ratios from a single rigid mold. A proof-of-principle measurement was carried out in isolated cardiac myocytes. The results conform to the physiologic behavior. This approach has the potential for evaluation of mechanical interaction between various biological units and the substrates while minimizing the fabrication cost and complexity.

Mechanical force is one of the most important parameters in cellular physiological behavior. To quantify the cellular force locally and more precisely, soft material probes, such as bulk polymeric surfaces or raised individual polymeric structures, have been developed which are deformable by the cell. The extent of deformation and the elastic properties of the probes allow for calculation of the mechanical forces exerted by the cell. Bulk polymeric surfaces have the disadvantage of requiring computational intensive calculations due to the continuous distortion of a large area, and investigators have attempted to address this problem by using raised polymeric structures to simplify the derivation of cellular mechanical force. These studies, however, have ignored the possibility of formation of local adhesions of the cell to the underlying base substrate, which could result in inaccurate cellular force measurements. Clearly, there is a need to develop polymeric structures that can efficiently isolate the cells from the underlying base substrate, in order to eliminate the continuous distortion problem. In this paper, we demonstrate the measurement of cellular force in isolated cardiac myocytes using single-spaced polymeric microstructures. Each structure is 2 µm in diameter and single-spaced packed. This geometry of the structures successfully isolates the cells from the underlying substrate. Displacement of the structures was measured in areas underneath the attached cell and at areas in close proximity to the cell. The results show that the individual structures underneath the cell were significantly displaced whereas no substantial strain in the underlying base substrate was detected. The mechanical force of the cell was derived from the displacements of individual structures upon multiplication with the locally determined spring constant. The force distribution reveals a parallel alignment as well as a periodic motion of the contractile units of the myocyte. The flexible fabrication methodology of the polymeric substrate and straightforward determination of minute forces provide a useful way to study cellular mechanical force.

Plasma-enhanced chemical-vapor deposited (PECVD) silane-based oxides (SiO_x) have been widely used in both microelectronics and microelectromechanical systems (MEMS) to form electrical and/or mechanical components. In this paper, a nanoindentation-based microbridge testing method is developed to measure both the residual stresses and Young’s modulus of PECVD SiO_x films on silicon wafers. Theoretically, we considered both the substrate deformation and residual stress in the thin film and derived a closed formula of deflection versus load. The formula fitted the experimental curves almost perfectly, from which the residual stresses and Young’s modulus of the film were determined. Experimentally, freestanding microbridges made of PECVD SiO_x films were fabricated using the silicon undercut bulk micromachining technique. Some microbridges were subjected to rapid thermal annealing (RTA) at a temperature of 400 °C, 600 °C, or 800 °C to simulate the thermal process in the device fabrication. The results showed that the as-deposited PECVD SiO_x films had a residual stress of –155 ± 17 MPa and a Young’s modulus of 74.8 ± 3.3 GPa. After the RTA, Young’s modulus remained relatively unchanged at around 75 GPa, however, significant residual stress hysteresis was found in all the films. A microstructure-based mechanism was then applied to explain the experimental results of the residual stress changes in the PECVD SiO_x films after the thermal annealing.

A single-crystal silicon micromachined air turbine supported on gas-lubricated bearings has been operated in a controlled and sustained manner at rotational speeds greater than 1 million revolutions per minute, with mechanical power levels approaching 5 W. The device is formed from a fusion bonded stack of five silicon wafers individually patterned on both sides using deep reactive ion etching (DRIE). It consists of a single stage radial inflow turbine on a 4.2-mm diameter rotor that is supported on externally pressurized hydrostatic journal and thrust bearings. This work presents the design, fabrication, and testing of the first microfabricated rotors to operate at circumferential tip speeds up to 300 m/s, on the order of conventional high performance turbomachinery. Successful operation of this device motivates the use of silicon micromachined high-speed rotating machinery for power microelectromechanical systems (MEMS) applications such as portable energy conversion, micropropulsion, and microfluidic pumping and cooling.

In this paper, an innovative method to create embedded microchannels is presented. The presented technology is based on a direct-write technique using a scanning laser system to pattern a single layered SU-8. The enormous flexibility of the scanning laser system can be seen in two key features: the laser pulsing can be controlled spot-by-spot with variable exposure doses, and the laser intensity penetrating into samples can be adjusted by varying the laser focus level. The UV laser direct-write method greatly simplifies the fabrication processes. Moreover, it can be set up in a conventional manufacturing environment without the need for clean room facilities. The second part of this paper describes the underlying theory and method to determine Young’s modulus of exposed SU-8 by using a laser acoustic microscopy system. The laser-based ultrasonic technique offers a non-contact, non-destructive means of evaluation and material characterization. This paper will determine Young’s modulus of UV exposed SU-8 generated with different exposure doses. Measurements show that Young’s modulus is highly dependent on exposure dose. Young’s modulus ranges from 3.8 to 5.4 GPa when the thickness of a fully cross-linked SU-8 microbeam varies from 100 to 205 µm with a gradually increased UV exposure dose.

The structural relaxation of plasma-enhanced chemical-vapor-deposited (PECVD) silane-based silicon oxide films during thermal cycling and annealing has been studied using wafer curvature measurements. These measurements, which determine stress in the amorphous silicon oxide films, are sensitive to both plastic deformation and density changes. A quantitative case study of such changes has been done based upon the experimental results. A microstructure-based mechanism elucidates seams as a source of density change and voids as a source of plastic deformation, accompanied by a viscous flow. This theory was then used to explain a series of experimental results that are related to thermal cycling as well as annealing of PECVD silicon oxide films including stress hysteresis generation and reduction and coefficient of thermal-expansion changes. In particular, the thickness effect was examined; PECVD silicon oxide films with a thickness varying from 1 to 40 µm were studied, as certain demanding applications in microelectromechanical systems require such thick films serving as heat∕electrical insulation layers.

In micro-satellites, delicate instruments are compacted into a limited space. This raises concerns of active cooling and remote cooling. Silicon based micro-pump arrays are employed thanks to manufacturing simplicity, a small cryogen charge, etc, and keep the instruments within a narrow cryogenic temperature range. The pumping capacity and reliability of the micro-pump are critical in terms of heat balance calculation and lifetime evaluation. The pumping capacity is associated with the diaphragm deflection while the reliability is associated with stress and fatigue. Both of them heavily depend on the silicon diaphragm, one of the key components. This paper examines the pumping capacity and reliability of the micro-pump under cryogenic temperature for micro-satellite applications. In this work, differential pressure was used for the actuation of a single-crystal silicon diaphragm. Diaphragm deflection and stress distribution were achieved using interferometry and micro-Raman spectroscopy, respectively. As a result, smaller pumping capacity was derived under cryogenic temperature, compared to that under room temperature, indicating a stiffer material. From stress mapping, the edge centers were believed to be the most vulnerable to fracture, which was further validated by analyzing the fracture diaphragm. Moreover, a fatigue testing was conducted for 1.8 million cycles with no damage found, verifying silicon as a viable material for long time operation in a cryogenic environment.

This letter introduces a three-dimensional manufacturing approach to the rapid processing of microfluidic structures using a scanning laser system. This technique takes advantage of the nonuniform distribution of laser power along its incident axis. The laser processing perpendicular to the specimen surface is realized by fine-tuning focus levels and laser intensity. A large number of microfluidic components such as cantilevered valves, embedded channels, and other shapes requiring gaps between layers are demonstrated in a single layer. With this process, a class of microstructures with designed-in functionalities can be developed.

Micromachines rotating at high speeds require low drag bearings with adequate load capacity and stability. Such bearings must be compatible with the capabilities of microfabrication technology. A self-acting (hydrodynamic) gas thrust bearing was designed, fabricated and tested on a silicon microturbine. Conventional thrust bearing design techniques were adapted from macroscale literature. Microbearing design charts are presented that relate bearing performance to geometry. Such bearings exhibit a design tradeoff between load bearing capability and maximum operating speed (as limited by instabilities). The specific geometry described herein was intended to replace externally pressurized, hydrostatic thrust bearings in an existing device (a 4-mm-diameter silicon microturbine), thus the hydrodynamic bearing design was constrained to be compatible in geometry and fabrication process. The final design consisted of 2.2-µm deep by 40-µm wide spiral grooves around the 700-µm diameter bearing. The bearings were fabricated in silicon with standard RIE and DRIE techniques. Test devices demonstrated lift-off and operation up to 450,000 rpm with a load capacity of 0.03 N. Measurements of load capacity and stiffness were consistent with the analysis.

Measurement of residual strain/stress in microstructures using a focused ion beam (FIB) moiré technique is demonstrated in this paper. This technique is selected based on advantages of the FIB system in nano-machining, in situ deposition, imaging, and fine adjustment. A nano-grating is directly written on the top of the microstructures by ion milling without any etch mask; the FIB moiré pattern is formed by the interference between a prepared specimen grating and raster scan lines. Effects of milling sequence, grating spacing and trench depth on the nano-grating structures and moiré fringes have been investigated. Strain evolution in microstructures during underlying sacrificial layer etching was studied. The sign of strain was derived from a rotation moiré technique. Moreover, a mass loading gauge with nano-gram resolution has been built. Since the local strain of a microstructure itself can be monitored during the process, the FIB moiré technique has many potential applications in the mechanical metrology of micro/nano-electro-mechanical-systems (MEMS/NEMS).

As part of an effort to develop a microscale gas turbine engine for power generation and micropropulsion applications, this paper presents the design, fabrication, experimental testing, and modeling of the combustion system. Two radial inflow combustor designs were examined; a single-zone arrangement and a primary and dilution-zone configuration. Both combustors were micromachined from silicon using deep reactive ion etching (DRIE) and aligned fusion wafer bonding. Hydrogen-air and hydrocarbon-air combustion were stabilized in both devices, each with chamber volumes of 191 mm³. Exit gas temperatures as high as 1800 K and power densities in excess of 1100 MW/m³ were achieved. For the same equivalence ratio and overall efficiency, the dual-zone combustor reached power densities nearly double that of the single-zone design. Because diagnostics in microscale devices are often highly intrusive, numerical simulations were used to gain insight into the fluid and combustion physics. Unlike large-scale combustors, the performance of the microcombustors was found to be more severely limited by heat transfer and chemical kinetics constraints. Important design trades are identified and recommendations for microcombustor design are presented.

This paper discusses thermo-mechanical behavior of plasma-enhanced chemical vapor deposited oxide films during and after post-deposition thermal cycling and annealing. A series of thermal cycling experiments were conducted with various types of oxide and nitride films to elucidate the control mechanism of intrinsic stress generation and to develop engineering solutions for improving reliability of microelectromechanical system fabrication processes. Tensile intrinsic stress generation was observed during thermal cycling and the depletion of hydrogen and the shrinkage of micro voids existing in the oxide films was postulated as a major control mechanism for the stress generation and was modeled by an energy-based formulation. Subsequent experiments indicated that annealing at high temperature could reduce this intrinsic tensile stress. Both stress generation and relaxation were modeled to guide the development of engineering solutions to maintain structural integrity and improve fabrication performance.

High-precision fabrication is indispensable for high-speed silicon micro-rotors for power MEMS applications so as to minimize the rotor imbalance that deteriorates the rotor performance. Etch variation of deep reactive ion etch (DRIE) process results in differences in rotor blade heights and thus rotor imbalance. A Fourier transform of the etch non-uniformity along the rotor circumference revealed the global etch variation across the wafer and local variations in etch rates depending on the concentration or proximity of the patterned geometry. Rotor imbalance arising from the global etch variation of DRIE process was estimated, which compared favorably to results obtained from spinning experiments. The global etch non-uniformity which culminates in rotor imbalance could be alleviated to 0.25% across a rotor of 4.2 mm diameter by optimizing the plasma chamber pressure. The developed DRIE recipe successfully reduced the rotor imbalance and thus enhanced the rotordynamic performance. The manufacturing processes presented herein are readily applicable to the constructions of other microstructures containing intricate geometries and large etched areas.

This paper presents the development of micro-fabricated “on-chip” polysilicon igniters and temperature sensors for the combustion system of a micro-gas turbine engine. We have reported the design and fabrication results of a novel through-wafer interconnect scheme that could greatly facilitate electrical contacts in multi-level MEMS devices by allowing direct electrical access to the backside of a wafer. This paper presents the results of a further effort that uses these interconnects to make electrical contacts to a thin-film polysilicon resistor so as to evaluate its ignition capability and its use as a wall temperature sensor for the micro-gas turbine engine. An application of the through-wafer interconnects to a concept demonstration of thin-film polysilicon resistive igniters for the micro-engine showed that it was possible to initiate combustion and locally raise the temperature of the igniter to 900 °C so long as the chip is thermally isolated. The results were found to be in good agreement with the predictions of an FEM thermal model. The possibility of using the resistors as temperature sensors is also examined. The non-linear variation of polysilicon resistivity with annealing temperatures due to complex effects resulting from dopant atom segregation, secondary grain growth and crystallographic relaxation reduced the operating range of the sensors to 450 °C.

This paper reports the investigation of low-temperature silicon wafer fusion bonding for MEMS applications. A bonding process utilizing annealing temperatures between 400 and 1100 °C was characterized. The silicon–silicon bonded interface was analyzed by infrared transmission (IT) and transmission electron microscopy (TEM) and the bond toughness was quantified by a four-point bending–delamination technique.

This paper reports development of a hydrocarbon-fueled micro-combustion system for a micro-scale gas turbine engine for power generation and micro-propulsion applications. A three-wafer catalytic combustor was fabricated and tested. Efficiencies in excess of 40% were achieved for ethylene–air and propane–air combustion. A fabrication process for a six-wafer catalytic combustor was developed and this device was successfully constructed.

his paper presents residual stress characterization and fracture analysis of thick plasma-enhanced chemical vapor deposition (PECVD) oxide films. The motivation for this work is to elucidate the factors contributing to residual stress, deformation and fracture of silicon oxide films so as to refine the microfabrication process for power microelectromechanical systems (MEMS) manufacturing. The stress–temperature behavior of PECVD oxide films during annealing was studied. Analyses of residual stress relaxation, intrinsic stress generation, and the large deformation response of wafers were carried out. Preliminary experimental observations and estimates of oxide fracture were also provided.

Multi-stack wafer bonding is one of the most promising fabrication techniques for creating three-dimensional (3D) microstructures. However, there are several bonding issues that have to be faced and overcome to build multilayered structures successfully. Among these are: (1) chemical residues on surfaces to be bonded originating from the fabrication processes prior to bonding; (2) increased stiffness due to multiple bonded wafers and/or thick wafers; (3) bonding tool effects; (4) defect propagation to other wafer-levels after high-temperature annealing cycles. The problems and the solutions presented here are readily applicable to any microelectromechanical systems project involving the fabrication of multi-stack structures of two or more wafers containing intricate geometries and large etched areas.

The ability to predict and control the influence of process parameters during silicon etching is vital for the success of most MEMS devices. In the case of deep reactive ion etching (DRIE) of silicon substrates, experimental results indicate that etch performance as well as surface morphology and post-etch mechanical behavior have a strong dependence on processing parameters. In order to understand the influence of these parameters, a set of experiments was designed and performed to fully characterize the sensitivity of surface morphology and mechanical behavior of silicon samples produced with different DRIE operating conditions. The designed experiment involved a matrix of 55 silicon wafers with radius hub flexure (RHF) specimens which were etched 10 min under varying DRIE processing conditions. Data collected by interferometry, atomic force microscopy (AFM), profilometry, and scanning electron microscopy (SEM), was used to determine the response of etching performance to operating conditions. The data collected for fracture strength was analyzed and modeled by finite element computation. The data was then fitted to response surfaces to model the dependence of response variables on dry processing conditions.

This paper reports solutions to the problem of profile control of narrow trenches in the vicinity of wider topographic features, as well as for etching high aspect ratio, anisotropic trenches with depths in the 300–500 µm range, and of widths between 12 to 18 µm. Additionally, specific operating conditions are discussed to address uniformity variations across dies with diameters in excess of 4200 µm.

This paper reports residual stress measurements and fracture analysis in thick tetraethylorthosilicate (TEOS) and silane-based plasma enhanced chemical vapor deposition (PECVD) oxide films. The measured residual stress depended strongly on thermal process parameters; dissolved hydrogen gases played an important role in governing intrinsic stress. The tendency to form cracks was found to be a strong function of film thickness and annealing temperature. Critical cracking temperature was predicted using mixed mode fracture mechanics, and the predictions provide a reasonable match to experimental observations. Finally, engineering solutions were demonstrated to overcome the problems caused by wafer bow and film cracks. The results of this study should be able to provide important insights for the design of fabrication processes for MEMS devices requiring high temperature processing of films.

As part of a program to develop a micro gas turbine engine capable of producing 10–50 W of electrical power in a package less than one cubic centimeter in volume, we present the design, fabrication, packaging, and experimental test results for the 6-wafer combustion system for a silicon microengine. Comprising the main nonrotating functional components of the engine, the device described measures 2.1 cm × 2.1 cm × 0.38 cm and is largely fabricated by deep reactive ion etching through a total thickness of 3800 µm. Complete with a set of fuel plenums, pressure ports, fuel injectors, igniters, fluidic interconnects, and compressor and turbine static airfoils, this structure is the first demonstration of the complete hot flow path of a multilevel micro gas turbine engine. The 0.195 cm³ combustion chamber is shown to sustain a stable hydrogen flame over a range of operating mass flows and fuel-air mixture ratios and to produce exit gas temperatures in excess of 1600 K. It also serves as the first experimental demonstration of stable hydrocarbon microcombustion within the structural constraints of silicon. Combined with longevity tests at elevated temperatures for tens of hours, these results demonstrate the viability of a silicon-based combustion system for micro heat engine applications.

This article reports the design, fabrication, and experimental demonstration of through-wafer interconnects capable of allowing direct electrical access to the interior of a multilevel microelectromechanical system device. The interconnects exploit the ability to conformally coat a high aspect ratio trench with a thick layer of tetraethylorthosilicate to isolate a through-wafer silicon plug that can provide electrical contact across two sides of a low resistivity wafer. They hold the potential of a tenfold reduction in the parasitic capacitance of previously reported through-wafer vias, and are shown to make reliable contacts to the back side of a polysilicon resistive element. The high temperature capability of the interconnects is also examined, however, their application is found to be limited to temperatures below 1000°C due to localized degradation near the isolating trenches.

In the present work we studied the depth of damage layer in machined silicon wafers that was incorporated with chemical etching using micro-Raman spectroscopy. Subsurface damage causes changes in the shape and intensity for the shoulder (450–570 cm⁻¹) of the most intense band (519 cm⁻¹) and the second band (300 cm⁻¹) regions of the Raman spectrum. Etching reduces the thickness of the damage layer and, hence, the intensities at the shoulder and the second band. The intensities at the shoulder and the second band become stable when the damage layer is completely etched out. The shoulder consists of two Gaussian profiles: the major and the minor. The band for the major profile is independent of etching depth, but the band for the minor profile shifts toward the longer wave numbers with increasing etching period until the damage layer is completely etched out. The depth of the damage layer is determined by the profiles of the shoulder and the second band and confirmed by the band shift of the minor profile. Transmission electron microscopy (TEM) further verified the results with respect to the depth of the damage layer. TEM observation showed that dislocations and stacking faults are responsible for the subsurface damage.

In comparison with conventional heat treatment, high-temperature rapid thermal annealing (RTA) in a radio frequency (RF) induction-heated system can reduce or eliminate residual stresses in thin films in a few seconds. In this work, changes in the stress level due to the RTA of polycrystalline silicon thin films were studied as a function of annealing time and temperature. The corresponding variations in the microstructure and surface layer of the thin films were experimentally investigated by a variety of analytical tools. The results suggest that the residual stress evolution during annealing is dominated by two mechanisms: 1) microstructure variations of the polysilicon thin film and 2) effects of a surface layer formed during the heat treatment. The fact that the microstructure changes are more pronounced in samples after conventional heat treatment implies that the effects of the formed surface layer may dominate the final state of the residual stress in the thin film.

In the present study, micro-rotating-structures for local measurements of residual stresses in a thin film were simulated by the finite element method (FEM). A sensitivity factor – the ratio of the deflection of the micro-structure to the normalized residual stress is introduced and tabulated from the FEM results. Thereafter, a formula to calculate the residual stress is given so that the residual stress can be easily evaluated from the deflection of the rotating beam. A variety of optimized micro-rotating-structures were then designed and fabricated to verify the FEM results. Residual stresses in both silicon nitride and polysilicon thin films were determined by this technique and compared with measurements by the wafer-curvature method. The two methods lead to comparable results. In addition, the micro-rotating-structures have the ability to measure spatially and locally a large range of residual tensile or compressive stresses.

In situ observations of buckling evolution of polysilicon microbeams during etch of the underneath sacrificial layer were carried out under an optical microscope. The surface geometry was obtained by AFM measurements. As the etching progressed, three buckling patterns were identified. Closed formulas were derived from theoretical analysis based on both boundary conditions: simply supported and clamped. The theory predicts either the buckling pattern for a given residual stress or the compressive stress level for a given buckling pattern. The residual stress evaluated from the buckling pattern agrees with that measured by the curvature method.

Rapid thermal annealing reduces stress in a very short time, compared to regular furnace annealing, and can be an effective method for relaxing residual stress in polysilicon thin films. In this work, the effects of regular furnace and high-temperature rapid thermal annealing (RTA) on the residual stress of LPCVD polysilicon thin films have been investigated. The as-deposited 0.5 µm thick polysilicon films have an initial compressive stress of about 340 MPa, and the residual stress is relaxed quickly after a few cycles of RTA at higher temperatures. The stress dependence on annealing time at temperatures of 900–1150 µC has been analysed. Using X-ray diffraction (XRD), micro-Raman spectroscopy and transmission electron microscopy (TEM), we have studied the changes in the microstructure of the thin films induced by the RTA during the stress relaxation. Furthermore, variations in the composition of the surface layer due to annealing have been characterized by X-ray photoelectron spectroscopy (XPS).

The 1000, 1150 and 1400 °C isothermal sections of the Ti–Al–Nb system have been determined using a diffusion couple technique. A series of ternary alloys in the single-phase or multi-phase regions has been employed to identify the phase relations shown in these isothermal sections. A new ternary phase γ₁ has been identified. Several of the binary phases were found to have extensive solubilities, which increased with increasing temperature. The binary compounds TiAl₃ and NbAl₃ form a continuous solid solution in the ternary system. Determination of these phase relations was accomplished through the use of electron microprobe analysis (EPMA), X-ray diffraction (XRD) and microstructure observations.

The relationships among the grand potential,G, the chemical potential, µ_i, and the effective chemical potential, µ_i, are geometrically demonstrated. An important equation concerning the relationship between the effective chemical potential, µ_i, and the molar free energy, Fm, was deduced. Use of the grand potential method enables calculation of the phase equilibria for the miscibility gap and different crystal structures in the ternary system.