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 G. R. Jaffe, G. R. Holdman, M. S. Jang, D. Feng, M. A. Kats, V. W. Brar, "Effect of dust and hot spots on the thermal stability of laser sails", Nano Letters (2023) [journal link] [preprint]
Abstract: Laser sails propelled by gigawatt-scale ground-based laser arrays have the potential to reach relativistic speeds, traversing the solar system in hours and reaching nearby stars in years. Here, we describe the danger interplanetary dust poses to the survival of a laser sail during its acceleration phase. We show through multiphysics simulations how localized heating from a single optically absorbing dust particle on the sail can initiate a thermal runaway process that rapidly spreads and destroys the entire sail. We explore potential mitigation strategies, including increasing the in-plane thermal conductivity of the sail to reduce the peak temperature at hot spots and isolating the absorptive regions of the sail that can burn away individually.
 J. King, C. Wan, T. J. Park, S. Deshpande, Z. Zhang, S. Ramanathan, M. A. Kats, "Electrically tunable VO2-metal metasurface for mid-infrared switching, limiting, and nonlinear isolation", arXiv:2303.09060 (2023) [preprint]
Abstract: We demonstrate an electrically controlled metal-VO2 metasurface for the mid-wave infrared that simultaneously functions as a tunable optical switch, an optical limiter with a tunable limiting threshold, and a nonlinear optical isolator with a tunable operating range. The tunability is achieved via Joule heating through the metal comprising the metasurface, resulting in an integrated optoelectronic device. As an optical switch, the device has an experimental transmission ratio of ~100 when varying the bias current. Operating as an optical limiter, we demonstrated tunability of the limiting threshold from 20 mW to 180 mW of incident laser power. Similar degrees of tunability are also achieved for nonlinear optical isolation, which enables asymmetric (nonreciprocal) transmission.
 H. Mei, G. Ren, B. Zhao, J. Salman, G. Y. Jung, H. Chen, S. Singh, A. S. Thind, J. Cavin, J. A. Hachtel, M. Chi, S. Niu, G. Joe, C. Wan, N. Settineri, S. J. Teat, B. C. Chakoumakos, J. Ravichandran, R. Mishra, M. A. Kats, "Colossal optical anisotropy from atomic-scale modulations", arXiv:2303.00041 (2023) [preprint]
Abstract: In modern optics, materials with large birefringence (Δn, where n is the refractive index) are sought after for polarization control (e.g. in wave plates, polarizing beam splitters, etc.), nonlinear optics and quantum optics (e.g. for phase matching and production of entangled photons), micromanipulation, and as a platform for unconventional light-matter coupling, such as Dyakonov-like surface polaritons and hyperbolic phonon polaritons. Layered “van der Waals” materials, with strong intra-layer bonding and weak inter-layer bonding, can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optic axis is out of the plane of the layers and the layers are weakly attached, making the anisotropy hard to access. Here, we demonstrate that a bulk crystal with subtle periodic modulations in its structure — Sr9/8TiS3 — is transparent and positive-uniaxial, with extraordinary index ne = 4.5 and ordinary index no = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS3, results in the formation of TiS6 trigonal-prismatic units that break the infinite chains of face-shared TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr subsequently occupy the TiS6 octahedral blocks to form highly oriented and polarizable electron clouds, which selectively boost the extraordinary index ne and result in record birefringence (Δn > 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive-index modulation and low optical losses.
 S. Rosas, K. A. Schoeller, E. Chang, H. Mei, M. A. Kats, K. W. Eliceiri, X. Zhao, F. Yesilkoy, "Metasurface-enhanced mid-infrared spectrochemical imaging of tissues", Advanced Materials 2301208 (2023) [journal link] [preprint]
Abstract: Label-free and nondestructive mid-infrared vibrational hyperspectral imaging is emerging as an important ex-vivo tissue analysis tool, providing spatially resolved biochemical information critical to understanding physiological and pathological processes. However, the chemically complex and spatially heterogeneous composition of tissue specimens and the inherently weak interaction of infrared light with biomolecules limit the analytical performance of infrared absorption spectroscopy. Here, we introduce an advanced mid-infrared spectrochemical tissue imaging modality using metasurfaces that support strong surface-localized electromagnetic fields to capture quantitative molecular maps of large-area murine brain-tissue sections. Our approach leverages polarization-multiplexed multi-resonance plasmonic metasurfaces to simultaneously detect many different functional biomolecules. The resulting surface-enhanced mid-infrared spectral imaging (SE-MIRSI) method eliminates the non-specific effects of bulk tissue morphology on the quantitative analysis of fingerprint spectra and improves the chemical selectivity. We show that the metasurface enhancement increases the retrieval of amide I and II absorption bands associated with secondary structures of proteins. Moreover, we demonstrate that plasmonic metasurfaces enhance the chemical contrast in infrared images and enable the detection of ultrathin tissue regions that are not otherwise visible to conventional mid-infrared spectral imaging. While we tested our approach on murine brain tissue sections, this chemical imaging method is well-suited for any tissue type, which significantly broadens the potential impacts of our method for both translational research and clinical histopathology.
 A. Koch, H. Mei, J. Rensberg, M. Hafermann, J. Salman, C. Wan, R. Wambold, D. Blaschke, H. Schmidt, J. Salfeld, S. Geburt, M. A. Kats, and C. Ronning, "Heavily doped zinc oxide with plasma frequencies in the telecommunication wavelength range", Adv. Photonics Res. 2200181 (2022) [journal link] [preprint]
Abstract: Heavy and hyper doping of ZnO by a combination of gallium (Ga) ion implantation using a focused ion beam (FIB) system and post-implantation laser annealing is demonstrated. Ion implantation allows for the incorporation of impurities with nearly arbitrary concentrations, and the laser-annealing process enables dopant activation close to or beyond the solid-solubility limit of Ga in ZnO. Heavily doped ZnO:Ga with free-carrier concentrations of ≈10^21 cm^−3, resulting in a plasma wavelength of 1.02 μm, which is substantially shorter than the telecommunication wavelength of 1.55 μm is demonstrated. Thus, this approach enables the control of the plasma frequency of ZnO from the far infrared down to 1.02 μm, thus, providing a promising plasmonic material for applications in this regime.
 Y. Xiao, L. Phuttitarn, T. M. Graham, C. Wan, M. Saffman, and Mikhail A. Kats, "Lateral beam shifts and depolarization upon oblique reflection from dielectric mirrors", arXiv:2211.13927 (2022) [preprint]
Abstract: Dielectric mirrors comprising thin-film multilayers are widely used in optical experiments because they can achieve substantially higher reflectance compared to metal mirrors. Here we investigate potential problems that can arise when dielectric mirrors are used at oblique incidence, in particular for focused beams. We found that light beams reflected from dielectric mirrors can experience lateral beam shifts, beam-shape distortion, and depolarization, and these effects have a strong dependence on wavelength, incident angle, and incident polarization. Because vendors of dielectric mirrors typically do not share the particular layer structure of their products, we designed and simulated several dielectric-mirror stacks, and then also measured the lateral beam shift from two commercial dielectric mirrors and one coated metal mirror. We hope that this paper brings awareness of the tradeoffs between dielectric mirrors and front-surface metal mirrors in certain optics experiments, and suggest that vendors of dielectric mirrors provide information about beam shifts, distortion, and depolarization when their products are used at oblique incidence.
 C. Yao, H. Mei, Y. Xiao, A. Shahsafi, W. Derdeyn, J. L. King, C. Wan, R. O. Scarlat, M. H. Anderson, and M. A. Kats, "Correcting thermal-emission-induced detector saturation in infrared spectroscopy", Optics Express 30, 38458 (2022) [journal link] [preprint]
Abstract: We found that temperature-dependent infrared spectroscopy measurements using a Fourier-transform spectrometer can have substantial errors, especially for elevated sample temperatures and collection using an objective lens (e.g., using an infrared microscope). These errors arise as a result of partial detector saturation due to thermal emission from the measured sample reaching the detector, resulting in nonphysical apparent reduction of reflectance or transmittance with increasing temperature. Here, we demonstrate that these temperature-dependent errors can be corrected by implementing several levels of optical attenuation that enable "convergence testing" of the measured reflectance or transmittance as the thermal-emission signal is reduced, or by applying correction factors that can be inferred by looking at the spectral regions where the sample is not expected to have a substantial temperature dependence.
 J. King, A. Shahsafi, Z. Zhang, C. Wan, Y. Xiao, C. Huang, Y. Sun, P. J. Roney, S. Ramanathan, M. A. Kats, “Wavelength-by-Wavelength Temperature-Independent Thermal Radiation Utilizing an Insulator–Metal Transition”, ACS Photonics 9, 2742 (2022) [journal link] [preprint]
Abstract: Both the magnitude and spectrum of the blackbody radiation distribution change with temperature. Here, we designed the temperature-dependent spectral emissivity of a coating to counteract all the changes in the blackbody radiation distribution over a certain temperature range, enabled by the nonhysteretic insulator-to-metal phase transition of SmNiO3. At each wavelength within the long-wave infrared atmospheric-transparency window, the thermal radiance of our coating remains nearly constant over a temperature range of at least 20 °C. Our approach can conceal thermal gradients and transient temperature changes from infrared imaging systems, including those that discriminate by wavelength, such as multispectral and hyperspectral cameras.
Abstract: Laser sail-based spacecraft—where a powerful Earth-based laser propels a lightweight outer-space vehicle—have been recently proposed by the Breakthrough Starshot Initiative as a means of reaching relativistic speeds for interstellar space travel. The laser intensity at the sail required for this task is at least 1 GW m^−2 and, at such high intensities, thermal management of the sail becomes a significant challenge even when using materials with low linear absorption coefficients. Silicon is proposed as one leading candidate material for the sail due to its low sub-bandgap absorption and high index of refraction, which allows for low-mass-density designs. However, here it is shown that the temperature-dependent linear absorption of silicon can lead to thermal runaway at temperatures above 400–500 K for even the most optimistic viable assumptions of the material quality. Additionally, above a design-specific threshold laser intensity, nonlinear two-photon absorption triggers thermal runaway regardless of initial temperature. Resonator-based designs, which concentrate the field, exhibit lower threshold intensities than geometries that minimize the electric field such as Bragg reflectors.
 P. Huft, Y. Song, T. M. Graham, K. Jooya, S. Deshpande, C. Fang, M. Kats, and M. Saffman, "Simple, passive design for large optical trap arrays for single atoms", Physical Review A 105, 063111 (2022) [journal link] [preprint]
Abstract: We present an approach for trapping cold atoms in a two-dimensional (2D) optical trap array generated with a 4f filtering scheme and custom transmission mask without any active device. The approach can be used to generate arrays of bright or dark traps, or both simultaneously with a single wavelength for forming two-species traps. We demonstrate the design by creating a 2D array of 1225 dark trap sites, where single Cs atoms are loaded into regions of near-zero intensity in an approximately Gaussian profile trap. Moreover, we demonstrate a simple solution to the problem of out-of-focus trapped atoms, which occurs due to the Talbot effect in periodic optical lattices. Using a high power yet low cost spectrally and spatially broadband laser, out-of-focus interference is mitigated, leading to near perfect removal of Talbot plane traps.
 Y. Xiao, M. Sheldon and M. A. Kats, "Super-Planckian emission cannot really be ‘thermal’", Nature Photonics 16, 397 (2022) [journal link]
Abstract: A heat-powered emitter can sometimes exceed the Planck thermal-emission limit. We clarify when such super-Planckian emission is possible, arguing that far-field super-Planckian emission requires a distribution of energy that is not consistent with a unique temperature, and therefore the process should not be called ‘thermal emission’.
 D. Feng, R. A. Wambold, Y. Xiao, C. Wan, Z. Yu, V. W. Brar, and M. A. Kats, "Comment on “Electromagnetic force on structured metallic surfaces”", Physical Review B 105, 207401 (2022) [journal link]
Abstract: In [Phys. Rev. B 92, 115416 (2015)], Velzen and Webb proposed that by structuring a metal film suspended in free space with a periodic array of nanoscale slots, the steady-state optical pressure on the film resulting from an incident plane wave can be substantially enhanced compared to the standard radiation-pressure limit. This result appears to be in violation of the law of momentum conservation, given that the momentum of free-space electromagnetic plane waves is well established. We recalculated the optical fields and forces for the same configuration as Velzen and Webb and found that, although the calculated optical fields and force densities agree with the published result, the resulting total optical forces were not enhanced and, instead, matched the standard radiation-pressure relation. Our calculations imply that the diverging results arise due to different choices of the region used for integration of the force density.
 B. Zhao, M. S. B. Hoque, G. Y. Jung, H. Mei, S. Singh, G. Ren, M. Milich, Q. Zhao, N. Wang, H. Chen, S. Niu, S. J. Lee, C. T. Kuo, J. S. Lee, J. A. Tomko, H. Wang, M. A. Kats, R. Mishra, P. E. Hopkins, J. Ravichandran, "Orientation Controlled Anisotropy in Single Crystals of Quasi-1D BaTiS3", Chemistry of Materials 34, 5680 (2022) [journal link] [preprint]
Abstract: Low-dimensional materials with chain-like (one-dimensional) or layered (two-dimensional) structures are of significant interest due to their anisotropic electrical, optical, thermal properties. One material with chain-like structure, BaTiS3 (BTS), was recently shown to possess giant in-plane optical anisotropy and glass-like thermal conductivity. To understand the origin of these effects, it is necessary to fully characterize the optical, thermal, and electronic anisotropy of BTS. To this end, BTS crystals with different orientations (a- and c-axis orientations) were grown by chemical vapor transport. X-ray absorption spectroscopy (XAS) was used to characterize the local structure and electronic anisotropy of BTS. Fourier transform infrared (FTIR) reflection/transmission spectra show a large in-plane optical anisotropy in the a-oriented crystals, while the c-axis oriented crystals were nearly isotropic in-plane. BTS platelet crystals are promising uniaxial materials for IR optics with their optic axis parallel to the c-axis. The thermal conductivity measurements revealed a thermal anisotropy of ~4.5 between the c- and a-axis. Time-domain Brillouin scattering showed that the longitudinal sound speed along the two axes is nearly the same suggesting that the thermal anisotropy is a result of different phonon scattering rates.
 Y. Xiao, M. A. Kats, J. J. Greffet, Q. Li, G. T. Papadakis, "Materials and Devices for Engineering of Thermal Light: feature issue introduction", Optical Materials Express 12, 1450 (2022) [journal link]
Abstract: Thermal radiation describes the emission of electromagnetic waves from hot objects. Although the basics of thermal radiation have been well understood for more than a century, engineering of thermal radiation is an active research field, in part because of applications to energy harvesting, lighting, and thermoregulation. The rapidly evolving research base sits at the intersection of materials science, photonics, and thermal physics. In eight research papers and one opinion paper, this feature issue of Optical Materials Express advances the multidisciplinary field of engineering of thermal light.
 W. B. Derdeyn, S. Mastromarino, R. Gakhar, M. H. Anderson, M. A. Kats, R. O. Scarlat, “Infrared optical spectroscopy of molten fluorides: methods, electronic and vibrational data, structural interpretation, and relevance to radiative heat transfer”, arXiv:2203.15757 (2022) [preprint]
Abstract: To help address the need for predicting radiative heat transfer (RHT) behavior of molten salts, we conducted a comprehensive review of methods and data from infrared optical spectroscopic measurements on molten fluoride salts. Transmittance, reflectance, and trans-reflectance experimental methods are discussed, along with the corresponding data reduction methodology and the limitations of each technique. Optical spectroscopy is a convenient indirect probe for changes in structural parameters with temperature and composition. Electronic and vibrational absorption data for transition-metal, lanthanide, and actinide solutes and vibrational absorption data for alkali and alkaline earth fluoride solvents are compiled, and the corresponding structural interpretation is discussed and compared with other experimental and theoretical work. We find that solvent and solute vibrational absorption can be significant in the mid-infrared, resulting in near-infrared edges of significance to RHT. Extrapolation and averaging of existing edge data leads to estimated gray absorption coefficient values at 700 ˚C of 493 m^-1 for FLiBe and 148 m^-1 for FLiNaK, both within the range of 1 – 6000 m^-1 identified to be of engineering relevance for radiative heat transfer analysis.
 H. Mei, A. Koch, C. Wan, J. Rensberg, Z. Zhang, J. Salman, M. Hafermann, M. Schaal, Y. Xiao, R. Wambold, S. Ramanathan, C. Ronning, M. A. Kats, “Tuning carrier density and phase transitions in oxide semiconductors using focused ion beams”, Nanophotonics, 11, 3923 (2022) [journal link] [preprint]
Abstract: We demonstrate spatial modification of the optical properties of thin-film metal oxides, zinc oxide and vanadium dioxide as representatives, using a commercial focused ion beam (FIB) system. Using a Ga+ FIB and thermal annealing, we demonstrated variable doping of a band semiconductor, zinc oxide (ZnO), achieving carrier concentrations from 10^18 cm^-3 to 10^20 cm^-3. Using the same FIB without subsequent thermal annealing, we defect-engineered a correlated semiconductor, vanadium dioxide (VO2), locally modifying its insulator-to-metal transition (IMT) temperature by range of ~25 °C. Such area-selective modification of metal oxides by direct writing using a FIB provides a simple, mask-less route to the fabrication of optical structures, especially when multiple or continuous levels of doping or defect density are required.
 C. Wan, D. Woolf, C. M. Hessel, J. Salman, Y. Xiao, C. Yao, A. Wright, J. M. Hensley, M. A. Kats, “Switchable induced-transmission filters enabled by vanadium dioxide”, Nano Letters 22, 6 (2022) [journal link] [preprint]
Abstract: An induced-transmission filter (ITF) uses an ultrathin metallic layer positioned at an electric-field node within a dielectric thin-film bandpass filter to select one transmission band while suppressing other bands that would have been present without the metal layer. We introduce a switchable mid-infrared ITF where the metal can be “switched on and off”, enabling the modulation of the filter response from a single band to multiband. The switching is enabled by the reversible insulator-to-metal phase transition of a subwavelength film of vanadium dioxide (VO2). Our work generalizes the ITF ─ a niche type of bandpass filter ─ into a new class of tunable devices. Furthermore, our fabrication process ─ which begins with thin-film VO2 on a suspended membrane ─ enables the integration of VO2 into any thin-film assembly that is compatible with physical vapor deposition processes and is thus a new platform for realizing tunable thin-film filters.
 M. Hafermann, R. Schock, C. Wan, J. Rensberg, M. A. Kats, and C. Ronning, “Fast recovery of ion-irradiation-induced defects in Ge2Sb2Te5 thin films at room temperature”, Optical Materials Express 11, 3535 (2021) [journal link] [preprint]
Abstract: Phase-change materials serve a broad field of applications ranging from non-volatile electronic memory to optical data storage by providing reversible, repeatable, and rapid switching between amorphous and crystalline states accompanied by large changes in the electrical and optical properties. Here, we demonstrate how ion irradiation can be used to tailor disorder in initially crystalline Ge2Sb2Te5 (GST) thin films via the intentional creation of lattice defects. We found that continuous Ar+-ion irradiation at room temperature of GST films causes complete amorphization of GST when exceeding 0.6 (for rock-salt GST) and 3 (for hexagonal GST) displacements per atom (n_dpa). While the transition from rock-salt to amorphous GST is caused by progressive amorphization via the accumulation of lattice defects, several transitions occur in hexagonal GST upon ion irradiation. In hexagonal GST, the creation of point defects and small defect clusters leads to the disordering of intrinsic vacancy layers (van der Waals gaps) that drives the electronic metal–insulator transition. Increasing disorder then induces a structural transition from hexagonal to rock-salt and then leads to amorphization. Furthermore, we observed different annealing behavior of defects for rock-salt and hexagonal GST. The higher amorphization threshold in hexagonal GST compared to rock-salt GST is caused by an increased defect-annealing rate, i.e., a higher resistance against ion-beam-induced disorder. Moreover, we observed that the recovery of defects in GST is on the time scale of seconds or less at room temperature.
Abstract: All spectrometers rely on some mechanism to achieve spectral selectivity; common examples include gratings, prisms, and interferometers with moving mirrors. We developed a new spectroscopic technique -- Planck spectroscopy -- that measures the spectral emissivity of a surface using only a temperature-controlled stage and a detector, without any wavelength-selective optical components. Planck spectroscopy involves the measurement of temperature-dependent thermally emitted power, where the spectral selectivity is realized via the temperature- and wavelength dependence of Planck's law. We experimentally demonstrated Planck spectroscopy in the mid infrared, for wavelengths from 3 to 13 um -- limited primarily by the bandwidth of our detector -- with resolution of approximately 1 um. The minimalistic setup of Planck spectroscopy can be implemented using infrared cameras to achieve low-cost infrared hyperspectral imaging and imaging ellipsometry.
 B. F. Bachman, Z. R. Jones, G. R. Jaffe, J. Salman, R. Wambold, Z. Yu, J. T. Choy, S. J. Kolkowitz, M. A. Eriksson, M. A. Kats, and R. J. Hamers, “High-Density Covalent Grafting of Spin-Active Molecular Moieties to Diamond Surfaces”, Langmuir 37, 30 (2021) [journal link]
Abstract: Functionalization of diamond surfaces with TEMPO and other surface paramagnetic species represents one approach to the implementation of novel chemical detection schemes that make use of shallow quantum color defects such as silicon-vacancy (SiV) and nitrogen-vacancy (NV) centers. Yet, prior approaches to quantum-based chemical sensing have been hampered by the absence of high-quality surface functionalization schemes for linking radicals to diamond surfaces. Here, we demonstrate a highly controlled approach to the functionalization of diamond surfaces with carboxylic acid groups via all-carbon tethers of different lengths, followed by covalent chemistry to yield high-quality, TEMPO-modified surfaces. Our studies yield estimated surface densities of 4-amino-TEMPO of approximately 1.4 molecules nm–2 on nanodiamond (varying with molecular linker length) and 3.3 molecules nm–2 on planar diamond. These values are higher than those reported previously using other functionalization methods. The ζ-potential of nanodiamonds was used to track reaction progress and elucidate the regioselectivity of the reaction between ethenyl and carboxylate groups and surface radicals.
 M. Zhou, D. Liu, S. W. Belling, H. Cheng, M. A. Kats, S. Fan, M. L. Povinelli, and Z. Yu , “Inverse Design of Metasurfaces Based on Coupled-Mode Theory and Adjoint Optimization”, ACS Photonics 8, 8 (2021) [journal link]
Abstract: Metasurfaces typically have sizes much larger than the wavelength yet contain a large number of subwavelength features. Thus, it is difficult to design entire metasurfaces using full-wave simulations. However, without full-wave simulations, most existing design approaches cannot accurately model the interactions between the individual elements comprising the metasurface. Here, we demonstrate an approach for the design of resonant metasurfaces based on coupled-mode theory. Our approach fully describes wave dynamics and coupling in metasurfaces and is much more computationally efficient than full-wave simulations. As an example, we show that the combination of coupled-mode theory and adjoint optimization can be used for the inverse design of high-numerical-aperture (0.9) metalenses with sizes as large as 10000 wavelengths. The computation efficiency of our approach is orders of magnitude faster than full-wave simulations. Complex functionalities such as angle-multiplexed metasurface holograms can also be realized. With its accuracy and efficiency, the proposed framework can be a powerful design tool for large-scale resonant flat-optics devices.
Abstract: Optical bottle beams can be used to trap atoms and small low-index particles. We introduce a figure of merit for optical bottle beams, specifically in the context of optical traps, and use it to compare optical bottle-beam traps obtained by three different methods. Using this figure of merit and an optimization algorithm, we identified optical bottle-beam traps based on a Gaussian beam illuminating a metasurface that are superior in terms of power efficiency than existing approaches. We numerically demonstrate a silicon metasurface for creating an optical bottle-beam trap.
 J. Siegel, J. Dwyer, A. Suresh, N. Safron, M. Fortman, C. Wan, J. Choi, W. Wei, V. Saraswat, W. Behn, M. A. Kats, M. Arnold, P. Gopalan, V. W. Brar, “Using Bottom-Up Lithography and Optical Nonlocality to Create Short-Wave Infrared Plasmonic Resonances in Graphene”, ACS Photonics 8, 5 (2021) [journal link] [preprint]
Abstract: Graphene plasmonic resonators have been broadly studied in the terahertz and mid-infrared ranges because of their electrical tunability and large confinement factors, which can enable the dramatic enhancement of light–matter interactions. In this work, we demonstrate that the characteristic scaling laws of resonant graphene plasmons change for smaller (<40 nm) plasmonic wavelengths and that those changes modify the optical confinement properties of graphene plasmonic resonators, allowing their operational frequency to be expanded into the short-wave infrared (SWIR). These effects are realized in centimeter-scale arrays of graphene resonators as narrow as 12 nm, which are created using a novel, bottom-up block copolymer lithography method. Measurements of these structures reveal that their plasmonic resonances are strongly influenced by nonlocal and quantum effects, which push their resonant frequency well into the SWIR (free-space wavelength ∼2.2 μm), 75% higher frequency than previous experimental works. The confinement factors of these resonators reach 137 ± 25, among the largest reported in literature for any type of 2D optical resonator. The combined SWIR response and large confinement of these structures make them an attractive platform to explore ultrastrongly enhanced spontaneous emission.
 J. Salman, C. A Stifler, A. Shahsafi, C.-Y. Sun, S. Weibel, M. Frising, B. E Rubio-Perez, Y. Xiao, C. Draves, R. A Wambold, Z. Yu, D. C. Bradley, G. Kemeny, Pupa UPA Gilbert, Mikhail A Kats, “Hyperspectral interference tomography of nacre”, Proceedings of the National Academy of Sciences 118, e2023623118 (2021) [journal link] [preprint]
Abstract: Structural characterization of biologically formed materials is essential for understanding biological phenomena and their enviro-nment, and for generating new bio-inspired engineering concepts. For example, nacre—the inner lining of some mollusk shells—encodes local environmental conditions throughout its formation and has exceptional strength due to its nanoscale brick-and-mortar structure. This layered structure, comprising alternating transparent aragonite (CaCO3) tablets and thinner organic polymer layers, also results in stunning interference colors. Existing methods of structural characterization of nacre rely on some form of cross-sectional analysis, such as scanning or transmission electron microscopy or polarization-dependent imaging contrast (PIC) mapping. However, these techniques are destructive and too time- and resource-intensive to analyze large sample areas. Here, we present an all-optical, rapid, and nondestructive imaging technique—hyperspectral interference tomography (HIT)—to spatially map the structural parameters of nacre and other disordered layered materials. We combined hyperspectral imaging with optical-interference modeling to infer the mean tablet thickness and its disorder in nacre across entire mollusk shells from red and rainbow abalone (Haliotis rufescens and Haliotis iris) at various stages of development. We observed that in red abalone, unexpectedly, nacre tablet thickness decreases with age of the mollusk, despite roughly similar appearance of nacre at all ages and positions in the shell. Our rapid, inexpensive, and nondestructive method can be readily applied to in-field studies.
Abstract: A radiative vapor condenser sheds heat in the form of infrared radiation and cools itself to below the ambient air temperature to produce liquid water from vapor. This effect has been known for centuries, and is exploited by some insects to survive in dry deserts. Humans have also been using radiative condensation for dew collection. However, all existing radiative vapor condensers must operate during the nighttime. Here, we develop daytime radiative condensers that continue to operate 24 h a day. These daytime radiative condensers can produce water from vapor under direct sunlight, without active consumption of energy. Combined with traditional passive cooling via convection and conduction, radiative cooling can substantially increase the performance of passive vapor condensation, which can be used for passive water extraction and purification technologies.
 J. Salman, M. K. Gangishetty, B. E. Rubio-Perez, D. Feng, Z. Yu, Z. Yang, C. Wan, M. Frising, A. Shahsafi, D. N. Congreve, M. A. Kats, "Passive frequency conversion of ultraviolet images into the visible using perovskite nanocrystals", Journal of Optics, 23, 054001 (2021) [journal link] [preprint]
Abstract: We demonstrate a passive down-conversion imaging system that converts broadband ultraviolet light to narrow-band green light while preserving the directionality of rays, and thus enabling direct down-conversion imaging. At the same time our system has high transparency in the visible, enabling superimposed visible and ultraviolet imaging. The frequency conversion is performed by a subwavelength-thickness transparent downconverter based on highly efficient CsPbBr3 nanocrystals incorporated into the focal plane of a simple telescope or relay-lens geometry. The resulting imaging performance of this down-conversion system approaches the diffraction limit. This demonstration sets the stage for the incorporation of other high-efficiency perovskite nanocrystal materials to enable passive multi-frequency conversion imaging systems.
Abstract: Optical limiters are nonlinear devices that feature decreasing transmittance with increasing incident optical intensity, and thus can protect sensitive components from high-intensity illumination. The ideal optical limiter reflects rather than absorbs light in its active (“limiting”) state, minimizing risk of damage to the limiter itself. Previous efforts to realize reflective (rather than absorbing) limiters were based on embedding nonlinear layers into relatively thick multilayer photonic structures, resulting in substantial fabrication complexity, reduced speed and, in some instances, limited working bandwidth. In this paper, these tradeoffs are overcome by using the insulator-to-metal transition (IMT) in vanadium dioxide (VO2) to achieve intensity-dependent modulation of resonant transmission through aperture antennas. Due to the large change of optical properties across the IMT, low-quality-factor resonators are sufficient to achieve high on–off ratios in the transmittance of the limiter. As a result, our ultrathin reflective limiter (thickness ≈1/100 of the free-space wavelength) is broadband in terms of operating wavelength (>2 µm at 10 µm) and angle of incidence (up to ≈50° away from the normal). Our analysis of the experimental results via opto-thermal simulations provides insight into limiter performance and is a useful guidance for further engineering efforts.
 R. A. Wambold, Z. Yu, Y. Xiao, B. Bachman, G. Jaffe, S. Kolkowitz, J. T. Choy, M. Eriksson, R. J. Hamers, M. A. Kats, "Adjoint-optimized nanoscale light extractor for nitrogen-vacancy centers in diamond", Nanophotonics 10, 1 (2020) [journal link] [preprint]
Abstract: We designed a nanoscale light extractor (NLE) for the efficient outcoupling and beaming of broadband light emitted by shallow, negatively charged nitrogen-vacancy (NV) centers in bulk diamond. The NLE consists of a patterned silicon layer on diamond and requires no etching of the diamond surface. Our design process is based on adjoint optimization using broadband time-domain simulations and yields structures that are inherently robust to positioning and fabrication errors. Our NLE functions like a transmission antenna for the NV center, enhancing the optical power extracted from an NV center positioned 10 nm below the diamond surface by a factor of more than 35, and beaming the light into a ±30° cone in the far field. This approach to light extraction can be readily adapted to other solid-state color centers.
 A. Shahsafi, J. Salman, B. E. Rubio Perez, Y. Xiao, C. Wan, M. A. Kats, “Infrared polarizer based on direct coupling to surface-plasmon polaritons”, Nano Letters 20, 8483 (2020) [journal link] [preprint]
Abstract: We propose a new type of reflective polarizer based on polarization-dependent coupling to surface plasmon polaritons (SPPs) from free space. This inexpensive polarizer is relatively narrowband but features an extinction ratio of up to 1000 with efficiency of up to 95% for the desired polarization (numbers from a calculation) and thus can be stacked to achieve extinction ratios of 10^6 or more. As a proof of concept, we experimentally realized a polarizer based on nanoporous aluminum oxide that operates around a wavelength of 10.6 μm, corresponding to the output of a CO2 laser, using aluminum anodization, a low-cost electrochemical process.
 Y. Xiao, C. Wan, A. Shahsafi, J. Salman, Z. Yu, R. Wambold, H. Mei, B. E. Rubio Perez, W. Derdeyn, C. Yao, M. A. Kats, “Precision measurements of temperature‐dependent and nonequilibrium thermal emitters”, Laser & Photonics Reviews 14, 1900443 (2020) [journal link] [preprint]
Abstract: Thermal emission is the radiation of electromagnetic waves from hot objects. The promise of thermal‐emission engineering for applications in energy harvesting, radiative cooling, and thermal camouflage has recently led to renewed research interest in this topic. However, accurate and precise measurements of thermal emission in a laboratory setting can be challenging in part due to the presence of background emission from the surrounding environment and the measurement instrument itself. This problem is especially acute for thermal emitters that have unconventional temperature dependence, operate at low temperatures, or are out of equilibrium. In this paper, general procedures are described, recommended, and demonstrated for thermal‐emission measurements that can accommodate such unconventional thermal emitters.
 M. A. Kats, "Thinking systematically about the online academic experience", IEEE Nanotechnology Magazine 14, 3 (2020) [link] [preprint]
Abstract: Reports on the challenges of online learning with the recent COVID-19 pandemic. Discusses the impact to both students and their teachers and presents ways in which students can get the best out of the academic experience in this environment.
Abstract: Dark-field microscopy is a widely used imaging method that emphasizes sharp edges and other small features, but typically requires specialized microscope components. Researchers have now engineered special substrates that enable dark-field microscopy using simple bright-field microscopes.
Abstract: The first online-only meeting in photonics, held on 13 January 2020, was a resounding success, with 1100 researchers participating remotely to discuss the latest advances in photonics. Here, the organizers share their tips and advice on how to organize an online conference.
 Y. Xiao, C. Wan, A. Shahsafi, J. Salman, M. A. Kats, "Depth thermography: non-invasive 3D temperature profiling using infrared thermal emission", ACS Photonics 7, 853 (2020) [preprint]
Abstract: We introduce a technique based on infrared thermal emission, termed depth thermography, that can remotely measure the temperature distribution beneath the surface of certain objects. Depth thermography utilizes the thermal-emission spectrum in the semitransparent spectral region of the target object to extract its temperature as a function of depth, in contrast with conventional thermography, which uses the spectrally integrated thermally emitted power to measure the surface temperature. Coupled with two-dimensional imaging, for example using an infrared hyperspectral camera or scanning a single-pixel spectrometer, this technique can yield volumetric temperature distributions. We carried out a proof-of-concept experiment on an asymmetrically heated fused-silica window, extracting the temperature distribution throughout the sample. Depth thermography may enable noncontact volumetric temperature measurements of microscopic objects such as multilayer electronic devices or macroscopic volumes of liquids and gases, as well as simultaneous all-optical measurements of optical and thermal properties of materials.
 Z. Yu, C. Wan, J. Salman, B. S. Gundlach, Y. Xiao, Z. Yu, M. A. Kats, “Optical components based on multi-refractive-index metamaterials”, Journal of Physics D: Applied Physics 53, 015108 (2020) [preprint]
Abstract: We studied optical components (lenses, prisms, Fabry–Perot-type etalons) comprising a metamaterial-like medium that cannot be described by a single set of refractive-index values, even for a fixed frequency, vacuum wavevector, and polarization. The metastructure that we explored is a periodic stack of dissimilar metal-clad waveguides with a subwavelength width and spacing, which guide light at different phase velocities. From the ray-optics perspective, this multi-refractive-index 'metamaterial' (MRIM) can be viewed as a spatial superposition of multiple homogeneous materials, each of which can be engineered independently. Using full-wave simulations, we demonstrate several optical components based on MRIMs, including triangular prisms that deflect light to multiple angles, lenses with multiple focal points, and multi-index Fabry–Perot etalons with an enhanced density of resonant modes. We also analytically derive the Fresnel-like reflection, transmission, and 'swapping' coefficients at the interfaces between MRIMs and conventional materials, which enable the design of MRIM-based optical structures.
Abstract: Conventional wisdom states that the hotter an object is, the brighter it glows. This is the case for thermal light at any wavelength and enables applications such as infrared imaging and noncontact thermometry. We demonstrate a coating that emits the same amount of thermal radiation irrespective of temperature, within a temperature range of about 30 °C. This is accomplished using samarium nickel oxide—a quantum material that changes strongly but gradually as a function of temperature. This is the first time that temperature-independent thermal radiation has been demonstrated, and has substantial implications for infrared camouflage, privacy shielding, and radiative heat transfer.
 A. Shahsafi, G. Joe, S. Brandt, A. V. Shneidman, N. Stanisic, Y. Xiao, R. Wambold, Z. Yu, J. Salman, J. Aizenberg, M. A. Kats, "Wide-angle spectrally selective absorbers and thermal emitters based on inverse opals", ACS Photonics 6, 2607 (2019) [preprint]
Abstract: Engineered optical absorbers are of substantial interest for applications ranging from stray light reduction to energy conversion. We demonstrate a large-area (centimeter-scale) metamaterial that features near-unity frequency-selective absorption in the mid-infrared wavelength range. The metamaterial comprises a self-assembled porous structure known as an inverse opal, here made of silica. The structure’s large volume fraction of voids, together with the vibrational resonances of silica in the mid-infrared spectral range, reduce the metamaterial’s refractive index to close to that of air and introduce considerable optical absorption. As a result, the frequency-selective structure efficiently absorbs incident light of both polarizations even at very oblique incidence angles. The absorber remains stable at high temperatures (measured up to ∼900 °C), enabling its operation as a frequency-selective thermal emitter. The excellent performance of this absorber/emitter and ease of fabrication make it a promising surface coating for passive radiative cooling, laser safety, and other large-area applications.
 Y. Xiao, N. A. Charipar, J. Salman, A. Pique, M. A. Kats, “Nanosecond mid-infrared pulse generation via modulated thermal emissivity”, Light: Science and Applications 8, 51 (2019) [preprint]
Abstract: We demonstrate the generation of nanosecond mid-infrared pulses via fast modulation of thermal emissivity enabled by the absorption of visible pump pulses in unpatterned silicon and gallium arsenide. The free-carrier dynamics in these materials result in nanosecond-scale modulation of thermal emissivity, which leads to nanosecond pulsed thermal emission. To our knowledge, the nanosecond thermal-emissivity modulation in this work is three orders of magnitude faster than what has been previously demonstrated. We also indirectly observed subnanosecond thermal pulses from hot carriers in semiconductors. The experiments are well described by our multiphysics model. Our method of converting visible pulses into the mid infrared using modulated emissivity obeys different scaling laws and can have significant wavelength tunability compared to approaches based on conventional nonlinearities.
 D. G. Baranov, Y. Xiao, I. A. Nechepurenko, A. Krasnok, A. Alù, M. A. Kats, “Nanophotonic engineering of far-field thermal emitters”, Nature Materials 18, 920 (2019) [preprint]
Abstract: Thermal emission is a ubiquitous and fundamental process by which all objects at non-zero temperatures radiate electromagnetic energy. This process is often assumed to be incoherent in both space and time, resulting in broadband, omnidirectional light emission toward the far field, with a spectral density related to the emitter temperature by Planck’s law. Over the past two decades, there has been considerable progress in engineering the spectrum, directionality, polarization and temporal response of thermally emitted light using nanostructured materials. This Review summarizes the basic physics of thermal emission, lays out various nanophotonic approaches to engineer thermal emission in the far field, and highlights several applications, including energy harvesting, lighting and radiative cooling.
 J. Siegel, A. Wang, S. G. Menabde, M. A. Kats, M. S. Jang, V. W. Brar, “Self-stabilizing laser sails based on optical metasurfaces”, ACS Photonics 6, 8, 2032 (2019) [preprint]
Abstract: This article investigates the stability of “laser sail”-style spacecraft constructed from dielectric metasurfaces with areal densities <1 g/m^2. We show that the microscopic optical forces exerted on a metasurface by a high-power laser can be engineered to achieve passive self-stabilization, such that it is optically trapped inside the drive beam and self-corrects against angular and lateral perturbations. The metasurfaces we study consist of a patchwork of beam-steering elements that reflect light at different angles and efficiencies. These properties are varied across the area of the metasurface, and we use optical force modeling tools to explore the behavior of several metasurfaces with different scattering properties as they interact with beams that have different intensity profiles. Finally, we use full-wave numerical simulation tools to extract the actual optical forces that would be imparted on Si/SiO2 metasurfaces consisting of more than 400 elements, and we compare those results to our analytical models. We find that, under first-order approximations, there are certain metasurface designs that can propel a “laser-sail”-type spacecraft in a stable manner.
 Z. Wang, S. Yi, A. Chen, T. S. Luk, A. James, J. Nogan, W. Ross, G. Joe, A. Shahsafi, K. X. Wang, M. A. Kats, Z. Yu, “Single-shot on-chip spectral sensors based on photonic crystal slabs”, Nature Communications 10, 1020 (2019) [peer review file]
Abstract: Miniaturized spectrometers have significant potential for portable applications such as consumer electronics, health care, and manufacturing. These applications demand low cost and high spectral resolution, and are best enabled by single-shot free-space-coupled spectrometers that also have sufficient spatial resolution. Here, we demonstrate an on-chip spectrometer that can satisfy all of these requirements. Our device uses arrays of photodetectors, each of which has a unique responsivity with rich spectral features. These responsivities are created by complex optical interference in photonic-crystal slabs positioned immediately on top of the photodetector pixels. The spectrometer is completely complementary metal–oxide–semiconductor (CMOS) compatible and can be mass produced at low cost.
 Y. Xiao, A. Shahsafi, C. Wan, P. J. Roney, G. Joe, Z. Yu, J. Salman, and M. A. Kats, “Measuring thermal emission near room temperature using Fourier-transform infrared spectroscopy”, Physical Review Applied 11, 014026 (2019) [preprint]
Abstract: Accurate characterization of thermal emitters can be challenging due to the presence of background thermal emission from components of the experimental setup and the surrounding environment. This is especially true for an emitter operating close to room temperature. Here, we explore the characterization of near-room-temperature thermal emitters using Fourier-transform infrared (FTIR) spectroscopy. We find that the thermal background arising from optical components placed between the beam splitter and the detector in an FTIR spectrometer appears as a “negative” contribution to the Fourier-transformed signal, leading to errors in thermal-emission measurements near room temperature. Awareness of this contribution will help properly calibrate low-temperature thermal-emission measurements.
 C. Wan, Z. Zhang, D. Woolf, C. M. Hessel, J. Rensberg, J. M. Hensley, Y. Xiao, A. Shahsafi, J. Salman, S. Richter, Y. Sun, M. M. Qazilbash, R. Schmidt-Grund, C. Ronning, S. Ramanathan, M. A. Kats, “On the optical properties of thin-film vanadium dioxide from the visible to the far infrared”, Annalen der Physik 1900188 (2019) [preprint] [dataset]
Abstract: The insulator‐to‐metal transition (IMT) in vanadium dioxide (VO2) can enable a variety of optics applications, including switching and modulation, optical limiting, and tuning of optical resonators. Despite the widespread interest in VO2 for optics, the wavelength‐dependent optical properties across its IMT are scattered throughout the literature, are sometimes contradictory, and are not available at all in some wavelength regions. Here, the complex refractive index of VO2 thin films across the IMT is characterized for free‐space wavelengths from 300 nm to 30 µm, using broadband spectroscopic ellipsometry, reflection spectroscopy, and the application of effective‐medium theory. VO2 films of different thicknesses are studied, on two different substrates (silicon and sapphire), and grown using different synthesis methods (sputtering and sol–gel). While there are differences in the optical properties of VO2 synthesized under different conditions, these differences are surprisingly small in the ≈2–11 µm range where the insulating phase of VO2 also has relatively low optical loss. It is anticipated that the refractive‐index datasets from this article will be broadly useful for modeling and design of VO2‐based optical and optoelectronic components, especially in the mid‐wave and long‐wave infrared.
 N. Antonellis, R. Thomas, M. A. Kats, I. Vitebskiy, T. Kottos, “Nonreciprocity in Photonic Structures with Phase-Change Components”, Physical Review Applied 11, 024046 (2019) [preprint]
Abstract: We consider the scattering problem for an asymmetric-composite photonic structure with a component experiencing a thermally driven phase transition. Using a numerical example, we show that if the heating is caused by the incident light, the transmittance can become highly asymmetric within a broad range of light intensities. This effect can be utilized for nonreciprocal light transport, optical limiting, or power switching.
 S. Niu, G. Joe, H. Zhao, Y. Zhou, T. Orvis, H. Huyan, J. Salman, K. Mahalingam, B. Urwin, J. Wu, Y. Liu, T. Tiwald, S. B. Cronin, B. M. Howe, M. Mecklenburg, R. Haiges, D. J. Singh, H. Wang, M. A. Kats, J. Ravichandran, “Giant optical anisotropy in a quasi-1D crystal”, Nature Photonics 12, 392 (2018) [news & views] [author correction]
Abstract: Optical anisotropy is a fundamental building block for linear and nonlinear optical components such as polarizers, wave plates, and phase-matching elements. In solid homogeneous materials, the strongest optical anisotropy is found in crystals such as calcite and rutile. Attempts to enhance anisotropic light–matter interaction often rely on artificial anisotropic micro/nanostructures (form birefringence). Here, we demonstrate rationally designed, giant optical anisotropy in single crystals of barium titanium sulfide (BaTiS3). This material shows an unprecedented, broadband birefringence of up to 0.76 in the mid- to long-wave infrared, as well as a large dichroism window with absorption edges at 1.6 μm and 4.5 μm for light with polarization along two crystallographic axes on an easily accessible cleavage plane. The unusually large anisotropy is a result of the quasi-one-dimensional structure, combined with rational selection of the constituent ions to maximize the polarizability difference along different axes.
 J. L. King, H. Jo, A. Shahsafi, K. Blomstrand, K. Sridharan, and M. A. Kats, “Impact of corrosion on the emissivity of advanced reactor structural alloys”, Journal of Nuclear Materials 508, 465 (2018) [preprint]
Abstract: Under standard operating conditions, the emissivity of structural alloys used for various components of nuclear reactors may evolve, affecting the heat transfer of the systems. In this study, mid-infrared emissivities of several reactor structural alloys were measured before and after exposure to environments relevant to next-generation reactors. We evaluated nickel-based alloys Haynes 230 and Inconel 617 exposed to helium gas at 1000 °C, nickel-based Hastelloy N and iron-based 316 stainless steel exposed to molten salts at 750–850 °C, 316 stainless steel exposed to liquid sodium at 650 °C, and 316 stainless steel and Haynes 230 exposed to supercritical CO2 at 650 °C. Emissivity was measured via emissive and reflective techniques using a Fourier transform infrared (FTIR) spectrometer. Large increases in emissivity are observed for alloys exposed to oxidizing environments, while only minor differences were observed in other exposure conditions.
 A. Shahsafi, Y. Xiao, J. Salman, B. S. Gundlach, C. Wan, P. J. Roney, and M. A. Kats, “Mid-infrared optics using dielectrics with refractive indices below unity”, Physical Review Applied 10, 034019 (2018) [preprint]
Abstract: Conventional transparent materials at optical frequencies have refractive-index values (n) greater than unity—most commonly between about 1 and 4. This paper explores optical phenomena made possible by using materials with refractive indices less than unity. We focus primarily on fused silica (SiO2), a well-studied dielectric with strong optical-phonon resonances in the mid infrared that result in a spectral region in which n < 1 with modest optical loss. Using this ubiquitous easy-to-deposit material, we demonstrate infrared-frequency external reflection, frustrated external reflection, and direct coupling to surface-plasmon polaritons from free space. Our work suggests that materials with refractive indices below unity can bring significant new functionalities to optical devices.
 Y. Sun, K. V. L. V. Narayanachari, C. Wan, X. Sun, H. Wang, K. A. Cooley, S. E. Mohney, D. White, A. Duwel, M. A. Kats, and S. Ramanathan, “Thermally tunable VO2-SiO2 nanocomposite thin-film capacitors”, Journal of Applied Physics 123, 114103 (2018)
Abstract: We present a study of co-sputtered VO2-SiO2 nanocomposite dielectric thin-film media possessing continuous temperature tunability of the dielectric constant. The smooth thermal tunability is a result of the insulator-metal transition in the VO2 inclusions dispersed within an insulating matrix. We present a detailed comparison of the dielectric characteristics of this nanocomposite with those of a VO2 control layer and of VO2/SiO2 laminate multilayers of comparable overall thickness. We demonstrated a nanocomposite capacitor that has a thermal capacitance tunability of ∼60% between 25 °C and 100 °C at 1 MHz, with low leakage current. Such thermally tunable capacitors could find potential use in applications such as sensing, thermal cloaks, and phase-change energy storage devices.
 C. Wan, E. Horak, J. King, J. Salman, Z. Zhang, Y. Zhou, P. Roney, B. Gundlach, S. Ramanathan, R. Goldsmith, and M. A. Kats, “Limiting optical diodes enabled by the phase transition of vanadium dioxide”, ACS Photonics 5, 2688 (2018) [preprint] [Q&A/comments]
Abstract: A limiting optical diode is an asymmetric nonlinear device that is bidirectionally transparent at low power but becomes opaque when illuminated by sufficiently intense light incident from a particular direction. We explore the use of a phase-transition material, vanadium dioxide (VO2), as an active element of limiting optical diodes. The VO2 phase transition can be triggered by optical absorption, resulting in a change in refractive index orders of magnitude larger than what can be achieved with conventional nonlinearities. As a result, a limiting optical diode based on incident-direction-dependent absorption in a VO2 layer can be very thin, and can function at low powers without field enhancement, resulting in broadband operation. We demonstrate a simple thin-film limiting optical diode comprising a transparent substrate, a VO2 film, and a semitransparent metallic layer. For sufficiently high incident intensity, our proof-of-concept device realizes broadband asymmetric transmission across the near-infrared, and is approximately ten times thinner than the free-space wavelength.
 D. M. Bierman, A. Lenert, M. A. Kats, Y. Zhou, S. Zhang, M. Ossa, S. Ramanathan, F. Capasso, and E. N. Wang, “Radiative thermal runaway due to negative differential thermal emission across a solid-solid phase transition”, Physical Review Applied 10, 021001 (2018) [preprint]
Abstract: Thermal runaway occurs when a rise in system temperature results in heat-generation rates exceeding dissipation rates. Here, we demonstrate that thermal runaway occurs in radiative (photon) systems given a sufficient level of negative-differential thermal emission. By exploiting the insulator-to-metal phase transition of vanadium dioxide, we show that a small increase in heat generation (e.g., 10 nW/mm^2) results in a large change in surface temperature (e.g., ∼35 K), as the thermal emitter switches from high emittance to low emittance. While thermal runaway is typically associated with catastrophic failure mechanisms, detailed understanding and control of this phenomenon may give rise to new opportunities in infrared sensing, camouflage, and rectification.
 J. Salman, M. Hafermann, J. Rensberg, C. Wan, R. Wambold, B. Gundlach, C. Ronning, M. A. Kats, “Flat optical and plasmonic devices using area-selective ion-beam doping of silicon”, Advanced Optical Materials 1701027 (2018) [cover]
Abstract: Highly doped semiconductors are an emerging platform for plasmonic devices. Unlike in noble metals, the carrier concentration of semiconductors can vary by many orders of magnitude, resulting in a widely tunable range of plasma wavelengths spanning the mid‐infrared and terahertz ranges. In this work, the potential of highly doped, ion‐beam‐patterned silicon is demonstrated as a fabrication‐friendly platform for flat optical devices. Detailed characterization of the optical properties of silicon is performed at various doping levels, and diffractive optical elements and plasmonic frequency‐selective surfaces that operate in the mid‐to‐far‐infrared regime are realized. The resulting optical devices are monolithic, flat, resilient to thermal and physical damage, and can be easily integrated into other silicon‐based platforms.
 B. S. Gundlach, M. Frising, A. Shahsafi, G. Vershbow, C. Wan, J. Salman, B. Rokers, L. Lessard, and M. A. Kats, “Design considerations for the enhancement of human color vision by breaking binocular redundancy”, Scientific Reports 8, 11971 (2018) [preprint]
Abstract: To see color, the human visual system combines the response of three types of cone cells in the retina—a compressive process that discards a significant amount of spectral information. Here, we present designs based on thin-film optical filters with the goal of enhancing human color vision by breaking its inherent binocular redundancy, providing different spectral content to each eye. We fabricated a set of optical filters that “splits” the response of the short-wavelength cone between the two eyes in individuals with typical trichromatic vision, simulating the presence of approximately four distinct cone types. Such an increase in the number of effective cone types can reduce the prevalence of metamers—pairs of distinct spectra that resolve to the same tristimulus values. This technique may result in an enhancement of spectral perception, with applications ranging from camouflage detection and anti-counterfeiting to new types of artwork and data visualization.
 J. Rensberg, Y. Zhou, S. Richter, C. Wan, S. Zhang, P. Schoppe, R. Schmidt-Grund, S. Ramanathan, F. Capasso, M. A. Kats, C. Ronning, “Epsilon-near-zero substrate engineering for ultra-thin-film perfect absorbers”, Physical Review Applied 8, 014009 (2017)
Abstract: Efficient suppression of reflection is a key requirement for perfect absorption of light. Recently, it has been shown that reflection can be effectively suppressed utilizing a single ultrathin film deposited on metals or polar materials featuring phonon resonances. The wavelength at which reflection can be fully suppressed is primarily determined by the nature of these substrates and is pinned to particular values near plasma or phonon resonances—the former typically in the ultraviolet or visible and the latter in the infrared. Here, we explicitly identify the required optical properties of films and substrates for the design of absorbing antireflection coatings based on ultrathin films. We find that completely suppressed reflection using films with thicknesses much smaller than the wavelength of light occurs within a spectral region where the real part of the refractive index of the substrate is n≲1, which is characteristic of materials with permittivity close to zero. We experimentally verify this condition by using an ultrathin vanadium dioxide film with dynamically tunable optical properties on several epsilon-near-zero materials, including aluminum-doped zinc oxide. By tailoring the plasma frequency of the aluminum-doped zinc oxide, we are able to tune the epsilon-near-zero point, thus achieving suppressed reflection and near-perfect absorption at wavelengths that continuously span the near-infrared and long-wave midinfrared ranges.
 M. Zhou, J. Liu, M. A. Kats, Z. Yu, “Optical Metasurface Based on the Resonant Scattering in Electronic Transitions”, ACS Photonics 4, 1279 (2017)
Abstract: Metasurfaces are an emerging platform for manipulating light on a two-dimensional plane. Existing metasurfaces comprise arrays of optical resonators such as plasmonic antennas or high-index nanorods. In this article, we describe a new approach to realize metasurfaces based on the resonant scattering in electronic transitions, such as in two-level systems (TLSs). These metasurfaces can reproduce all of the major results in conventional metasurfaces. In addition, since TLSs can be easily tunable and are orders of magnitude smaller than optical resonators, TLS metasurfaces can realize functions that are difficult to achieve with optical resonators.
 Z. Zhang, F. Zuo, C. Wan, A. Dutta, J. Kim, J. Rensberg, R. Nawrodt, H. H. Park, T. Larrabee, X. Guan, Y. Zhou, S.M. Prokes, C. Ronning, V. M. Shalaev, A. Boltasseva, M. A. Kats, and S. Ramanathan, “Evolution of metallicity in vanadium dioxide by creation of oxygen vacancies”, Physical Review Applied 7, 034008 (2017)
Abstract: Tuning of the electronic state of correlated materials is key to their eventual use in advanced electronics and photonics. The prototypical correlated oxide (VO2) is insulating at room temperature and transforms to a metallic state when heated to 67 °C (340 K). We report the emergence of a metallic state that is preserved down to 1.8 K by annealing thin films of VO2 at an ultralow oxygen partial pressure (PO2 ∼10^−24 atm). The films can be reverted back to their original state by annealing in oxygen, and this process can be iterated multiple times. The metallic phase created by oxygen deficiency has a tetragonal rutile structure and contains a large number of oxygen vacancies far beyond the solubility at equilibrium (greater than approximately 50 times). The oxygen starvation reduces the oxidation state of vanadium from V4+ to V3+ and leads to the metallization. The extent of resistance reduction (concurrent with tuning of optical properties) can be controlled by the time-temperature envelope of the annealing conditions since the process is diffusionally driven. This experimental platform, which can extensively tune oxygen vacancies in correlated oxides, provides an approach to study emergent phases and defect-mediated adaptive electronic and structural phase boundary crossovers.
 J. C. W. Song and M. A. Kats, "Giant Hall Photoconductivity in Narrow-Gapped Dirac Materials", Nano Letters 16, 7346 (2016)
Abstract: Carrier dynamics acquire a new character in the presence of Bloch-band Berry curvature, which naturally arises in gapped Dirac materials (GDMs). Here, we argue that photoresponse in GDMs with small band gaps is dramatically enhanced by Berry curvature. This manifests in a giant and saturable Hall photoconductivity when illuminated by circularly polarized light. Unlike Hall motion arising from a Lorentz force in a magnetic field, which impedes longitudinal carrier motion, Hall photoconductivity arising from Berry curvature can boost longitudinal carrier transport. In GDMs, this results in a helicity-dependent photoresponse in the Hall regime, where photoconductivity is dominated by its Hall component. We find that the induced Hall conductivity per incident irradiance is enhanced by up to 6 orders of magnitude when moving from the visible regime (with corresponding band gaps) to the far infrared. These results suggest that narrow-gap GDMs are an ideal test-bed for the unique physics that arise in the presence of Berry curvature and open a new avenue for infrared and terahertz optoelectronics.
 M. A. Kats and F. Capasso, "Optical absorbers based on strong interference in ultra-thin films", Laser & Photonics Reviews 10 (5), 699 (2016)
Abstract: Optical absorbers find uses in a wide array of applications across the electromagnetic spectrum, including photovoltaic and photochemical cells, photodetectors, optical filters, stealth technology, and thermal light sources. Recent efforts have sought to reduce the footprint of optical absorbers, conventionally based on graded structures or Fabry‐Perot‐type cavities, by using emerging concepts in plasmonics, metamaterials, and metasurfaces. Unfortunately, these new absorber designs require patterning on subwavelength length scales, and are therefore impractical for many large‐scale optical and optoelectronic devices.
In this article, we summarize recent progress in the development of optical absorbers based on lossy films with thicknesses significantly smaller than the incident optical wavelength. These structures have a small footprint and require no nanoscale patterning. We outline the theoretical foundation of these absorbers based on “ultra‐thin‐film interference”, including the concepts of loss‐induced phase shifts and critical coupling, and then review several applications, including ultra‐thin color coatings, decorative photovoltaics, high‐efficiency photochemical cells, and infrared scene generators.
 J. Rensberg, S. Zhang, Y. Zhou, A. S. McLeod, C. Schwarz, M. Goldflam, M. Liu, J. Kerbusch, R. Nawrodt, S. Ramanathan, D. N. Basov, F. Capasso, C. Ronning, M. A. Kats, "Active Optical Metasurfaces Based on Defect-Engineered Phase-Transition Materials", Nano Letters 16, 1050 (2016)
Abstract: Active, widely tunable optical materials have enabled rapid advances in photonics and optoelectronics, especially in the emerging field of meta-devices. Here, we demonstrate that spatially selective defect engineering on the nanometer scale can transform phase-transition materials into optical metasurfaces. Using ion irradiation through nanometer-scale masks, we selectively defect-engineered the insulator-metal transition of vanadium dioxide, a prototypical correlated phase-transition material whose optical properties change dramatically depending on its state. Using this robust technique, we demonstrated several optical metasurfaces, including tunable absorbers with artificially induced phase coexistence and tunable polarizers based on thermally triggered dichroism. Spatially selective nanoscale defect engineering represents a new paradigm for active photonic structures and devices.
2015 and earlier
 M. Khorasaninejad, F. Aieta, P. Kanhaiya, M. A. Kats, P. Genevet, D. Rousso, and F. Capasso, "Achromatic metasurface lens at telecommunications wavelengths", Nano Letters 15, 5358 (2015) [journal link] [supplementary]
 S. Zhang, M. A. Kats, Y. Cui, Y. Zhou, Y. Yao, S. Ramanathan, and F. Capasso, "Current-modulated optical properties of vanadium dioxide thin films in the phase transition region", Applied Physics Letters 105, 211104 (2014) [journal link] [supplementary]
 Y. Yao, R. Shankar, M. A. Kats, Y. Song, J. Kong, M. Loncar, and F. Capasso, "Electrically Tunable Metasurface Perfect Absorbers for Ultrathin Mid-Infrared Optical Modulators", Nano Letters 14, 6526 (2014) [journal link] [supplementary]
 M. A. Kats and F. Capasso, "Ultra-thin optical interference coatings on rough and flexible substrates", Applied Physics Letters 105, 131108 (2014) [journal link] [supplementary]
 M. A. Kats, R. Blanchard, S. Ramanathan, and F. Capasso, "Thin-Film Interference in Lossy, Ultra-Thin Layers", Optics and Photonics News, January (2014) [journal link]
 H. Gudjonson, M. A. Kats, K. Liu, Z. Nie, E. Kumacheva, and F. Capasso, "Accounting for inhomogeneous broadening in nano-optics by electromagnetic modeling based on Monte Carlo methods", Proceedings of the National Academy of Sciences 111, E639 (2014) [journal link] [supplementary]
 D. Woolf, M. A. Kats, and F. Capasso, "Spoof surface plasmon waveguide forces", Optics Letters 39, 517 (2014) [journal link]
 Y. Yao, M. A. Kats, R. Shankar, Y. Song, J. Kong, M. Loncar, and F. Capasso, "Wide wavelength tuning of optical antennas on graphene with nanosecond response time", Nano Letters 14, 214 (2014), published online in 2013. [journal link] [supplementary]
 F. Aieta, P. Genevet, M. A. Kats, and F. Capasso, "Aberrations of flat lenses and aplanatic metasurfaces", Optics Express 21, 31530 (2013) [journal link]
 M. A. Kats, R. Blanchard, S. Zhang, P. Genevet, C. Ko, S. Ramanathan, and F. Capasso, "Vanadium dioxide as a natural disordered metamaterial: perfect thermal emission and large broadband negative differential thermal emittance", Physical Review X 3, 041004 (2013) [journal link]
 J. Lin, P. Genevet, M. A. Kats, N. Antoniou, and F. Capasso, "Nanostructured holograms for broadband manipulation of vector beams", Nano Letters 13, 4269 (2013) [journal link]
 M. A. Kats, S. Byrnes, R. Blanchard, M. Kolle, P. Genevet, J. Aizenberg, and F. Capasso, "Enhancement of the color contrast in ultra-thin highly-absorbing optical coatings", Applied Physics Letters 103, 101104 (2013) [journal link]
 R. Blanchard, T. S. Mansuripur, B. Gokden, N. Yu, M. Kats, P. Genevet, K. Fujita, T. Edamura, M. Yamanishi, and F. Capasso, "High-power low-divergence tapered quantum cascade lasers with plasmonic collimators", Applied Physics Letters 102, 191114 (2013) [journal link]
 P. Genevet, J. Dellinger, R. Blanchard, A. She, M. Petit, B. Cluzel, M. A. Kats, F. De Fornel, and F. Capasso, "Generation of two-dimensional plasmonic bottle beams", Optics Express 21, 10295 (2013) [journal link]
 Y. Yao, M. A. Kats, P. Genevet, N. Yu, Y. Song, J. Kong, and F. Capasso, "Broad electrical tuning of graphene-loaded plasmonic antennas", Nano Letters 13, 1257 (2013) [journal link] [supplementary]
 N. Yu, P. Genevet, F. Aieta, M. A. Kats, R. Blanchard, G. Aoust, J.-P. Tetienne, Z. Gaburro, and F. Capasso, "Flat optics: controlling wavefronts with optical antenna metasurfaces", IEEE Selected Topics in Quantum Electronics (2013) [journal link]
 M. A. Kats, R. Blanchard, P. Genevet, J. Lin, D. Sharma, Z. Yang, M. M. Qazilbash, D. Basov, S. Ramanathan, and F. Capasso, "Thermal tuning of mid-infrared plasmonic antenna arrays using a phase change material", Optics Letters 38, 368 (2013) [journal link]
 P. Genevet, J. Lin, M. A. Kats, F. Capasso, "Holographic detection of the orbital angular momentum of light with plasmonic photodiodes", Nature Communications 3, 1278 (2012). [journal link] [supplementary]
 F. Aieta, A. Kabiri, P. Genevet, N. Yu, M. A. Kats, Z. Gaburro, and F. Capasso, "Reflection and refraction of light from metasurfaces with phase discontinuities", Journal of Nanophotonics 6, 063532 (2012). [journal link]
 N. Yu, F. Aieta, P. Genevet, M. A. Kats, Z. Gaburro, F. Capasso, "A broadband, background-free quarter-wave plate based on plasmonic metasurfaces", Nano Letters 12, 6328 (2012). [journal link] [supplementary]
 M. A. Kats, D. Sharma, J. Lin, P. Genevet, R. Blanchard, Z. Yang, M. M. Qazilbash, D. Basov, S. Ramanathan, and F. Capasso, "Ultra-thin perfect absorber using a tunable phase change material", Applied Physics Letters 101, 221101 (2012). [cover article] [journal link] [supplementary]
 M. A. Kats, R. Blanchard, P. Genevet and F. Capasso, "Nanometre optical coatings based on strong interference effects in highly absorbing media", Nature Materials 12, 20 (2013); published online in 2012. [journal link] [supplementary]
 F. Aieta, P. Genevet, M. A. Kats, N. Yu, R. Blanchard, Z. Gaburro, F. Capasso, "Aberration-free ultrathin flat lenses and axicons at telecom wavelengths based on plasmonic metasurfaces", Nano Letters 12, 4932 (2012) [journal link] [supplementary]
 M. A. Kats, P. Genevet, G. Aoust, N. Yu, R. Blanchard, F. Aieta, Z. Gaburro, and F. Capasso, "Giant birefringence in optical antenna arrays with widely tailorable optical anisotropy", Proceedings of the National Academy of Sciences 109, 12364 (2012) [journal link] [supplementary]
 R. Blanchard, G. Aoust, P. Genevet, N. Yu, M. A. Kats, Z. Gaburro, F. Capasso, "Modeling nanoscale V-shaped antennas for the design of optical phased arrays", Physical Review B 85, 155457 (2012) [journal link]
 F. Aieta, P. Genevet, N. Yu, M. A. Kats, Z. Gaburro, F. Capasso, "Out-of-plane reflection and refraction of light by anisotropic optical antenna metasurfaces with phase discontinuities", Nano Letters 12, 1702 (2012) [journal link] [supplementary]
 P. Genevet, N. Yu, F. Aieta, J. Lin, M. A. Kats, R. Blanchard, M. O. Scully, Z. Gaburro, F. Capasso, "Ultra-thin plasmonic optical vortex plate based on phase discontinuities", Applied Physics Letters 100, 13101 (2012) [cover article] [journal link]
 R. Blanchard, S. V. Boriskina, P. Genevet, M. A. Kats, J.-P. Tetienne, N. Yu, M. O. Scully, L. Dal Negro, F. Capasso, "Multi-wavelength mid-infrared plasmonic antennas with single nanoscale focal point", Optics Express 19, 22113 (2011) [journal link]
 M. A. Kats, N. Yu, P. Genevet, Z. Gaburro, F. Capasso, "Effect of radiation damping on the spectral response of plasmonic components", Optics Express 19, 21749 (2011) [journal link]
 P. Genevet, J.-P. Tetienne, R. Blanchard, M. A. Kats, J. P. B. Muller, M. O. Scully, F. Capasso, "Enhancement of optical processes in coupled plasmonic nanocavities", Applied Optics 50, 56 (2011) [journal link]
 N. Yu, P. Genevet, M. A. Kats, F. Aieta, Jean-Philippe Tetienne, F. Capasso, Z. Gaburro, "Light propagation with phase discontinuities: Generalized laws of reflection and refraction", Science 334, 333 (2011) [cover article] [journal link] [supplementary]
 M. A. Kats, D. Woolf, R. Blanchard, N. Yu, F. Capasso, "Spoof plasmon analogue of metal-insulator-metal waveguides", Optics Express 19, 14860 (2011) [journal link]
 J-P Tetienne, R. Blanchard, N. Yu, P. Genevet, M. A. Kats, J. A. Fan, T. Edamura, S. Furuta, M. Yamanishi, F. Capasso, "Dipolar modeling and experimental demonstration of multi-beam plasmonic collimators", New Journal of Physics 13, 53057 (2011) [journal link]
 N. Yu, Q. J. Wang, M. A. Kats, J. A. Fan, F. Capasso, S. P. Khanna, L. Li, A. G. Davies, E. H. Linfield, "Terahertz plasmonics", Electronics Letters 46, 52 (2010) [journal link]
 D. J. Lipomi, R. V. Martinez, M. A. Kats, S. H. Kang, P. Kim, J. Aizenberg, F. Capasso, G. M. Whitesides, "Patterning the tips of optical fibers with metallic nanostructures using nanoskiving", Nano Letters 11, 2 (2010) [journal link]
 P. Genevet, J. P. Tetienne, E. Gatzogiannis, R. Blanchard, M. A. Kats, M. O. Scully, F. Capasso, "Large enhancement of nonlinear optical phenomena by plasmonic nanocavity gratings", Nano Letters 10, 4880 (2010) [journal link] [supplementary]
 N. Yu, Q. J. Wang, M. A. Kats, J. A. Fan, S. P. Khanna, L. Li, A. G. Davies, E. H. Linfield, F. Capasso, "Designer spoof surface plasmon structures collimate terahertz laser beams", Nature Materials 9, 730 (2010) [journal link]
 D. J. Lipomi, M. A. Kats, P. Kim, S. H. Kang, J. Aizenberg, F. Capasso and G. M. Whitesides, "Fabrication and replication of arrays of single- or multicomponent nanostructures by replica molding and mechanical sectioning", ACS Nano 4, 4017 (2010) [cover article] [journal link] [supplementary]
 N. Yu, M. A. Kats, C. Pflugl, M. Geiser, Q. J. Wang, M. A. Belkin, F. Capasso, M. Fischer, A. Wittmann, J. Faist, T. Edamura, S. Furuta, M. Yamanishi, and H. Kan, "Multi-beam multi-wavelength semiconductor lasers", Applied Physics Letters 95, 161108 (2009) [cover article] [journal link]
 F. Ahmad, D. Tseng, M. Kats, F. Rana, "Energy limits imposed by two-photon absorption for pulse amplification in high power semiconductor optical amplifiers", Optics Letters 33, 1041 (2008) [journal link]