{"id":53,"date":"2021-07-13T13:44:09","date_gmt":"2021-07-13T17:44:09","guid":{"rendered":"https:\/\/sites.bu.edu\/ramachandranlab\/?page_id=53"},"modified":"2026-01-22T08:40:05","modified_gmt":"2026-01-22T13:40:05","slug":"research","status":"publish","type":"page","link":"https:\/\/sites.bu.edu\/ramachandranlab\/research\/","title":{"rendered":"Research"},"content":{"rendered":"
\"Examples
Examples of higher-order light beams with singularities. Structured light can have exotic diffractive and dispersive properties in free space and optical fibers, such as the ability to self-heal or carry orbital angular momentum.<\/figcaption><\/figure>\n
\"Video
Video depicting calcium activity of living neurons in the mouse cortex. Next generation microscopes leveraging ultra-fast, nonlinear interactions of structured light beams can improve penetration depth and information throughput for neuroscience.<\/figcaption><\/figure>\n
\"Research
Spatiotemporal nonlinear dynamics of ultra-fast pulses in highly multi-mode optical fibers. Nonlinear fiber optics exploiting the spatial degree of freedom \u2013 i.e. interactions between fiber modes \u2013 opens up new applications in quantum information, high power lasers, and biomedical imaging, amongst many others.<\/figcaption><\/figure>\n


Physics & Applications of Singular Light
\n<\/span><\/h3>\n

When we think of light, we think of a Gaussian-shaped spot, usually traveling in a straight line except when it encounters interfaces. This is, however, only the first, fundamental solution of the wave equation \u2013 much as a guitar string can have many modes (and frequencies) of vibrations, so can light also exist in different eigenstates. These peculiarly shaped light beams are characterized by singularities \u2013 i.e., regions in which some quantity does not have a well-defined value \u2013 in polarization, phase or amplitude. The physics of propagation of such beams reveals exotic effects, such as the ability of the beam to self-heal past obstructions (Bessel beams<\/strong>), the ability to carry orbital angular momentum (OAM)<\/strong> that makes light travel in helical paths rather than a straight line, or even the possibility of retaining memory<\/strong> of the paths that it takes. Analogous to the physics of tornadoes as well as electron orbitals, these beams reveal several unique classical <\/strong>and quantum<\/strong> properties not normally observed with conventional Gaussian beams. We study these fundamental properties and also apply them to varied applications such as high-capacity classical communications<\/strong> that consume low energy per bit<\/strong>, high-speed metrology including object sensing for autonomous systems<\/strong> as well as spectrometry <\/strong>and imaging<\/strong>.<\/p>\n

Talk by Prof. Ramachandran on Topologically Confined Modes<\/a><\/p>\n

\nRepresentative Publications<\/strong><\/summary>\n
    \n
  1. D. I. Shahar, H. B. Kabag\u00f6z, and S. Ramachandran “Generation of spatial combs digitized by orbital angular momentum<\/a><\/span>,” APL Photonics<\/em>\u00a0 9,<\/strong>\u00a0016113 (2024).<\/span><\/li>\n
  2. Z. Ma, P. Kristensen and S. Ramachandran, \u201cScaling information pathways in optical fibers by topological confinement<\/a>,\u201d Science<\/em><\/strong> 380<\/strong>, 278-282 (2023).<\/span><\/li>\n
  3. A.D. White, L. Su, D.I. Shahar, K.Y. Yang, G.Ho Ahn, J.L. Skarda, S. Ramachandran, J. Vu\u010dkovi\u0107, “Inverse Design of Optical Vortex Beam Emitters<\/a><\/span>,” ACS Photonics<\/em> 10<\/strong>, 803 (2023).<\/span><\/li>\n
  4. Z. Ma and S. Ramachandran, \u201cPropagation stability in optical fibers: role of path memory and angular momentum<\/a>,\u201d Nanophotonics<\/em> 10<\/strong>, 209 (2021).<\/li>\n
  5. A. P. Greenberg, G. Prabhakar, and S. Ramachandran, \u201cHigh resolution spectral metrology leveraging topologically enhanced optical activity in fibers<\/a>,\u201d Nature Communications<\/em> 11<\/strong>, 5257 (2020).<\/li>\n
  6. P. Gregg, P. Kristensen, A. Rubano, S. Golowich, L. Marrucci and S. Ramachandran, \u201cEnhanced Spin Orbit Interaction of Light in Highly Confining Optical Fibers for Mode Division Multiplexing<\/a>,\u201d Nature Communications<\/em> 10<\/strong>, 4707 (2019).<\/li>\n
  7. D. Cozzolino, D. Bacco, B.D. Lio, K. Ingerslev, Y. Ding, K. Dalgaard, P. Kristensen, M. Galili, K. Rottwitt, S. Ramachandran, L.K. Oxenl\u00f8we, \u201cOrbital Angular Momentum States Enabling Fiber-based High-dimensional Quantum Communication<\/a>,\u201d Phys. Rev. Applied<\/em> 11<\/strong>, 064058 (2019).<\/li>\n
  8. D. L. P. Vitullo, C.C. Leary, P. Gregg, R.A. Smith, D.V. Reddy, S. Ramachandran and M. G. Raymer, \u201cObservation of Interaction of Spin and Intrinsic Orbital Angular Momentum of Light<\/a>,” Phys. Rev. Lett.<\/em> 118<\/strong>, 083601 (2017).<\/li>\n
  9. B. N. Tugchin, N. Janunts, M. Steinert, K. Dietrich, D. Sivun, S. Ramachandran, K.V. Nerkararyan, A. T\u00fcnnermann, T. Pertsch, \u201cControlling the excitation of radially polarized conical plasmons in plasmonic tips in liquids<\/a>,\u201d RSC Advances<\/em>, 6<\/strong>, 53273 (2016).<\/li>\n
  10. N. Bozinovic, Y. Yue, Y. Ren, M. Tur, P. Kristensen, H Huang, A.E. Willner, and S. Ramachandran, \u201cTerabit-Scale Orbital Angular Momentum Mode Division Multiplexing in Fibers<\/a><\/span>,\u201d Science<\/em><\/strong> 340<\/strong>, 1545 (2013).<\/li>\n<\/ol>\n<\/details>\n

    Spatio-temporal & Ultra-fast Microscopy<\/span><\/h3>\n

    Optical imaging techniques such as fluorescence, confocal, or multiphoton microscopy, have allowed scientists to study the properties and function of living biological tissues<\/strong>, such as the mammalian brain<\/strong>. However, refractive index inhomogeneity in living tissue leads to scattering of input light fields, which reduces the penetration depth at which useful information about the sample can be gleaned. We aim to study how tailoring of nonlinear interactions between ultra-fast pulses<\/strong> of light \u2013 both inside and outside of biological samples \u2013 can be used to enhance signals from deep within the tissue<\/strong>. Furthermore, we study how the use of structured light<\/strong> \u2013 i.e.<\/em> light with spatial profiles more complex than a conventional Gaussian beam \u2013 can be used to improve information throughput in biological imaging in order to develop state-of-the-art microscopy methods for neuroscience<\/strong> and life science at large.<\/p>\n

    \nRepresentative Publications<\/strong><\/summary>\n
      \n
    1. J. Demas, J. Manley, F. Tejera, K. Barber, H. Kim, F. M. Traub, B. Chen, and A. Vaziri, \u201cHigh-speed, cortex-wide volumetric recording of neuroactivity at cellular resolution using light beads microscopy<\/a>,\u201d Nature Methods <\/em>18<\/strong>, 1103 (2021).<\/li>\n
    2. S. Weisenburger, F. Tejera, J. Demas, et al.<\/em>, \u201cVolumetric Ca2+<\/sup> Imaging in the Mouse Brain Using Hybrid Multiplexed Sculpted Light Microscopy<\/a>,\u201d Cell<\/em> 177<\/strong>, 1050 (2019).<\/li>\n
    3. L. Rish\u00f8j, B. Tai, P. Kristensen, and S. Ramachandran, \u201cSoliton self-mode conversion: revisiting Raman scattering of ultrashort pulses<\/a>,\u201d\u00a0Optica<\/em>\u00a06<\/strong>, 304 (2019).<\/li>\n
    4. L. Yan, P. Kristensen, S. Ramachandran, \u201cVortex fibers for stimulated emission depletion (STED) microscopy<\/a>,\u201d\u00a0APL Photonics<\/em>\u00a04<\/strong>, 022903 (2019).<\/li>\n
    5. L. Yan, P. Gregg, E. Karimi, A. Rubano, L. Marrucci, R. Boyd and S. Ramachandran, \u201cQ-plate enabled spectrally diverse orbital-angular-momentum conversion for STED microscopy<\/a>,\u201d\u00a0Optica<\/em> 2<\/strong>, 900 (2015).<\/li>\n<\/ol>\n<\/details>\n

      Nonlinear & Quantum Photonics<\/span><\/h3>\n

      Nonlinear optical phenomena represent the interaction of light with the material in which it propagates, resulting in myriad effects such as the ability to controllably alter its color and shape in both time and space. Because such effects perturb the principle of superposition in linear optics, nonlinear optics is one of the best known means of generating quantum entanglement<\/strong>. We study new nonlinear optical phenomena enabled by singular and structured light beams, such as new modal selection rules for Raman & Brillouin scattering<\/strong> and the ability to obtain hyper- <\/strong>or hybrid-quantum entanglement<\/strong>. We then exploit these unique effects for applications such as single-photon frequency conversion for quantum networks<\/strong>, and for developing high-power lasers<\/strong> in the long-pulse as well as ultra-fast<\/strong> regime, for myriad uses ranging from biomedical imaging<\/strong> to LIDAR-based sensing <\/strong>and underwater communications<\/strong>.<\/p>\n

      Short Course by Prof. Ramachandran on Multimode Nonlinearities<\/a><\/p>\n

      \nRepresentative Publications<\/strong><\/summary>\n
        \n
      1. X. Liu, D.I. Shahar, D.B. Kim, V.O. Lorenz, and S. Ramachandran, “Generation and engineering of orbital angular momentum biphotons in optical fibers<\/a><\/span>,” Optica Quantum<\/em> 3<\/strong>, 280-288 (2025).<\/span><\/li>\n
      2. Aku Antikainen and Siddharth Ramachandran, “Fundamental limits on fiber-based electron acceleration-and how to overcome them<\/a><\/span>,” J. Opt. Soc. Am. B<\/em> 42<\/strong>, 587-596 (2025).<\/span><\/li>\n
      3. Ilaria Cristiani et al<\/em>, \u201cRoadmap on multimode photonics<\/a>,\u201d Invited Paper<\/strong>, Opt.<\/em> 24<\/strong>, 083001 (2022).<\/span><\/li>\n
      4. X. Liu, Z. Ma, A. Antikainen, S. Ramachandran, \u201cRaman gain control in optical fibers with orbital-angular-momentum-induced chirality of light<\/a>,\u201d Opt. Express<\/em> 30<\/strong>, 26967-26974 (2022).<\/span><\/li>\n
      5. R. Lindberg, X. Liu, A. Zukauskas, S. Ramachandran and V. Pasiskevicius, “Simultaneous nonlinear wavelength and mode conversion for high-brightness blue sources<\/a>,” J. Opt. Soc. Am. B<\/em> 38<\/strong>, 3491 (2021).<\/li>\n
      6. X. Liu, E.N. Christensen, K. Rottwitt and S. Ramachandran, \u201cNonlinear four-wave mixing with enhanced diversity and selectivity via spin and orbital angular momentum conservation<\/a>,\u201d APL Photonics<\/em> 5<\/strong>, 010802 (2020).<\/li>\n
      7. G. Prabhakar, P. Gregg, L. Rishoj, P. Kristensen, and S. Ramachandran, “Octave-wide supercontinuum generation of light-carrying orbital angular momentum<\/a>,” Opt. Express 27<\/strong>, 11547-11556 (2019).<\/li>\n
      8. L. Rish\u00f8j, B. Tai, P. Kristensen, and S. Ramachandran, “Soliton self-mode conversion: revisiting Raman scattering of ultrashort pulses<\/a>,” Optica<\/em> 6<\/strong>, 304-308 (2019).<\/li>\n
      9. M. Ordu, J. Guo, G.Ng Pack, P. Shah, S. Ramachandran, M.K. Hong, L.D. Ziegler, S.N. Basu and S. Erramilli, \u201cNonlinear optics in germanium mid-infrared fiber material: Detuning oscillations in femtosecond mid-infrared spectroscopy<\/a>,\u201d AIP Advances<\/em> 7<\/strong>, 095125 (2017).<\/li>\n
      10. J. Demas, P. Steinvurzel, B. Tai, L. Rish\u00f8j, Y. Chen, and S. Ramachandran, \u201cIntermodal nonlinear mixing with Bessel beams in optical fiber<\/a>,\u201d Optica <\/em>2<\/strong>, 14 (2015).<\/span><\/li>\n<\/ol>\n<\/details>\n","protected":false},"excerpt":{"rendered":"

        Physics & Applications of Singular Light When we think of light, we think of a Gaussian-shaped spot, usually traveling in a straight line except when it encounters interfaces. This is, however, only the first, fundamental solution of the wave equation \u2013 much as a guitar string can have many modes (and frequencies) of vibrations, so […]<\/p>\n","protected":false},"author":19518,"featured_media":0,"parent":0,"menu_order":2,"comment_status":"closed","ping_status":"closed","template":"page-templates\/no-sidebars.php","meta":[],"_links":{"self":[{"href":"https:\/\/sites.bu.edu\/ramachandranlab\/wp-json\/wp\/v2\/pages\/53"}],"collection":[{"href":"https:\/\/sites.bu.edu\/ramachandranlab\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/sites.bu.edu\/ramachandranlab\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/sites.bu.edu\/ramachandranlab\/wp-json\/wp\/v2\/users\/19518"}],"replies":[{"embeddable":true,"href":"https:\/\/sites.bu.edu\/ramachandranlab\/wp-json\/wp\/v2\/comments?post=53"}],"version-history":[{"count":50,"href":"https:\/\/sites.bu.edu\/ramachandranlab\/wp-json\/wp\/v2\/pages\/53\/revisions"}],"predecessor-version":[{"id":1083,"href":"https:\/\/sites.bu.edu\/ramachandranlab\/wp-json\/wp\/v2\/pages\/53\/revisions\/1083"}],"wp:attachment":[{"href":"https:\/\/sites.bu.edu\/ramachandranlab\/wp-json\/wp\/v2\/media?parent=53"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}