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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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%.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.