Publications

Quasi-bound states in the continuum (QBICs) are Fano resonant states with long optical lifetimes controlled by symmetry-breaking perturbations. While conventional Fano responses are limited to linear polarizations and do not support tailored phase control, here we introduce QBICs born of chiral perturbations that encode arbitrary elliptical polarization states and enable geometric phase engineering. We thereby design metasurfaces with ultrasharp spectral features that shape the impinging wave front with near-unity efficiency. Our findings extend Fano resonances beyond their conventional limits, opening opportunities for nanophotonics, classical and quantum optics, and acoustics.

Photonic crystal slabs (PCSs) are a well-studied class of devices known to support optical Fano resonances for light normally incident to the slab, useful for narrow-band filters, modulators, and nonlinear photonic devices. In shallow-etched PCSs the linewidth of the resonances is easily controlled by tuning the etching depth. This design strength comes at the cost of large device footprints due to the poor in-plane localization of optical energy. In fully-etched PCSs realized in high-index-contrast material systems, the in-plane localization is greatly improved, but the command over linewidth suffers. This disadvantage in fully-etched PCSs, also known as high contrast gratings (HCGs), can be overcome by accessing symmetry-protected bound states in the continuum (BICs). By perturbing an HCG, the BIC may be excited from the free space with quality factor showing an inverse squared dependence on the magnitude of the perturbation, while inheriting the excellent in-plane localization of their unperturbed counterparts. Here, we report an exhaustive catalog of the selection rules (if and to which free space polarization coupling occurs) of symmetry-protected BICs controlled by in-plane symmetry breaking in six types of two-dimensional PCS lattices. The chosen lattices allow access to the three highest symmetry mode classes of unperturbed square and hexagonal PCSs. The restriction to in-plane symmetry breaking allows for manufacturing devices with simple lithographic fabrication techniques in comparison to out-of-plane symmetry breaking, useful for practical applications. The approach reported provides a high-level road map for designing PCSs supporting controllable sharp spectral features with minimal device footprints using a mature fabrication platform. To demonstrate the use of the resulting alphabet of structures, we numerically demonstrate nonlocal metasurface platforms for terahertz generation, mechanically tunable optical lifetimes, and wavefront shaping exclusively at resonance.

Strong field confinement, long-lifetime resonances, and slow-light effects suggest that metasurfaces are a promising tool for nonlinear optical applications. These nanostructured devices have been utilized for relatively high efficiency solid-state high-harmonic generation platforms, four-wave mixing, and Raman scattering experiments, among others. Here, we report the first all-dielectric metasurface to enhance harmonic generation from a surrounding gas, achieving as much as a factor of 45 increase in the overall yield for Argon atoms. When compared to metal nanostructures, dielectrics are more robust against damage for high power applications such as those using atomic gases. We employ dimerized high-contrast gratings fabricated in silicon-on-insulator that support bound states in the continuum, a resonance feature accessible in broken-symmetry planar devices. Our 1D gratings maintain large mode volumes, overcoming one of the more severe limitations of earlier device designs and greatly contributing to enhanced third- and fifth-harmonic generation. The interaction lengths that can be achieved are also significantly greater than the 10’s of nm to which earlier solid-state designs were restricted. We perform finite-difference time-domain simulations to fully characterize the wavelength, linewidth, mode profile, and polarization dependence of the resonances. Our experiments confirm these predictions and are consistent with other nonlinear optical properties. The tunable wavelength dependence and quality factor control we demonstrate in these devices make them an attractive tool for the next generation of high-harmonic sources, which are anticipated to be pumped at longer wavelengths and with lower peak power, higher repetition rate lasers.

Actively tunable and reconfigurable wavefront shaping by optical metasurfaces poses a significant technical challenge often requiring unconventional materials engineering and nanofabrication. Most wavefront-shaping metasurfaces can be considered “local” in that their operation depends on the responses of individual meta-units. In contrast, “nonlocal” metasurfaces function based on the modes supported by many adjacent meta-units, resulting in sharp spectral features but typically no spatial control of the outgoing wavefront. Recently, nonlocal metasurfaces based on quasi-bound states in the continuum have been shown to produce designer wavefronts only across the narrow bandwidth of the supported Fano resonance. Here, we leverage the enhanced light-matter interactions associated with sharp Fano resonances to explore the active modulation of optical spectra and wavefronts by refractive-index tuning and mechanical stretching. We experimentally demonstrate proof-of-principle thermo-optically tuned nonlocal metasurfaces made of silicon and numerically demonstrate nonlocal metasurfaces that thermo-optically switch between distinct wavefront shapes. This meta-optics platform for thermally reconfigurable wavefront shaping requires neither unusual materials and fabrication nor active control of individual meta-units.

Diffractive photonic devices manipulate light via local and nonlocal optical modes. Local devices, such as metasurfaces, can shape a wave front at multiple selected wavelengths, but inevitably modify light across the spectrum; nonlocal devices, such as grating filters, offer great frequency selectivity but limited spatial control. Here, we introduce a rational design paradigm using quasibound states in the continuum to realize multifunctional nonlocal devices: metasurfaces that produce narrow band spatially tailored wave fronts at multiple selected wavelengths and yet are otherwise transparent.

Adaptive control of broadband light is essential for diverse applications including building energy management and light modulation. Here, we present porous polymer coatings (PPCs), whose optical transmittance changes upon reversible wetting with common liquids, as a platform for optical management from solar to thermal wavelengths. In the solar wavelengths, reduction in optical scattering upon wetting changes PPCs from reflective to transparent. For poly(vinylidene fluoride-co-hexafluoropropene) PPCs, this corresponds to solar and visible transmittance changes of up to 0.74 and 0.80, respectively. For infrared (IR) transparent polyethylene PPCs, wetting causes an “icehouse-to-greenhouse” transition where solar transparency rises but thermal IR transparency falls. These performances are either unprecedented or rival or surpass those of notable optical switching (e.g., electrochromic and thermochromic) paradigms, making PPCs promising for large-scale optical and thermal management. Specifically, switchable sub-ambient radiative cooling (by 3.2°C) and above-ambient solar heating (by 21.4°C), color-neutral daylighting, and thermal camouflage are demonstrated.

Metasurfaces are optically thin metamaterials that promise complete control of the wavefront of light but are primarily used to control only the phase of light. Here, we present an approach, simple in concept and in practice, that uses meta-atoms with a varying degree of form birefringence and rotation angles to create high-efficiency dielectric metasurfaces that control both the optical amplitude and phase at one or two frequencies. This opens up applications in computer-generated holography, allowing faithful reproduction of both the phase and amplitude of a target holographic scene without the iterative algorithms required in phase-only holography. We demonstrate all-dielectric metasurface holograms with independent and complete control of the amplitude and phase at up to two optical frequencies simultaneously to generate two- and three-dimensional holographic objects. We show that phase-amplitude metasurfaces enable a few features not attainable in phase-only holography; these include creating artifact-free two-dimensional holographic images, encoding phase and amplitude profiles separately at the object plane, encoding intensity profiles at the metasurface and object planes separately, and controlling the surface textures of three-dimensional holographic objects.

Passive daytime radiative cooling (PDRC) involves spontaneously cooling a surface by reflecting sunlight and radiating heat to the cold outer space. Current PDRC designs are promising alternatives to electrical cooling but are either inefficient or have limited applicability. We present a simple, inexpensive, and scalable phase inversion–based method for fabricating hierarchically porous poly(vinylidene fluoride-co-hexafluoropropene) [P(VdF-HFP)HP] coatings with excellent PDRC capability. High, substrate-independent hemispherical solar reflectances (0.96 ± 0.03) and long-wave infrared emittances (0.97 ± 0.02) allow for subambient temperature drops of ~6°C and cooling powers of ~96 watts per square meter (W m−2) under solar intensities of 890 and 750 W m−2, respectively. The performance equals or surpasses those of state-of-the-art PDRC designs, and the technique offers a paint-like simplicity.

Metasurfaces and planar photonic crystals are two classes of subwavelength diffractive optical devices offering novel functionalities. The former employ independently operating subwavelength “meta-units” as their building blocks, while the latter exploit the collective response of many periodic building blocks. High contrast gratings (HCGs) are an example of one-dimensional (1D) planar photonic crystals with large refractive index contrast, exhibiting large in-plane scattering even with a limited number of grating periods. They are best known for their broadband features. Low contrast gratings (LCGs) are known for their control over sharp spectral features but require many periods due to small in-plane scattering. We explore a class of symmetry-broken HCGs called dimerized high contrast gratings (DHCGs), which have a period-doubling perturbation applied. DHCGs support modes accessible by free-space illumination with a long, controllable photon lifetime (inversely proportional to the magnitude of the perturbation) and reduced lateral energy divergence (confined by the high index contrast of the grating). We catalogue and clarify the resonant modes introduced by the dimerizing perturbation in 1D DHCGs and briefly explore the increased in-plane scattering present in two-dimensional (2D) DHCGs. We introduce an approach maximizing lateral localization by band structure engineering in the unperturbed HCG and using the dimerizing perturbation to generate sharp spectral features in devices with small footprint. We confirm the simultaneous control of photon lifetime and lateral localization with full-wave simulations of finite-sized DHCGs. We conclude by numerically demonstrating two compact devices (an optical modulator and a refractive index sensor) benefitting from the unique design freedoms of DHCGs.

Metamaterials have been designed to achieve a wide range of functionalities. Metamaterial absorbers are of particular interest for various applications such as infrared detectors, emissivity coatings, and photovoltaic cells. Various metamaterial platforms have been demonstrated to achieve perfect absorption and several attempts have been made to extend the absorption bandwidth of such devices. We demonstrate a broadband infrared absorber using an asymmetric Fabry–Perot cavity consisting of a monolithically fabricated two-layer metasurface. Superoctave optical absorption is achieved by tailoring the structure of the metasurface layers and the thickness of the cavity. The device yields absorptance of over 80% from λ = 4–16 μm, while maintaining the performance over a wide range of incident angles. In contrast to most metamaterial absorbers, our metasurface layers are made of customized indium tin oxide (ITO), conferring the advantage of CMOS compatibility compared to previous approaches using noble metals.