In this work, we introduce actively tunable PCM-FP (Fabry-Perot) and PCM-PNA (Plasmonic Nanohole Array) bandpass filters that possess high-speed tunability (MHz), narrow spectral bandwidth, high-transmissivity, broad tuning range, in an all solid-state design in a wide variety of imaging and spectroscopic applications. We also present the results from a Materials International Space Station Experiment (MISSE-14) in which chalcogenide phase change material (PCM) optical components are exposed and tested in Low Earth Orbit to determine their suitability for space applications. Our samples including Ge2Sb2Te5, Ge2Sb2Se4Te1, Sb2S3 thin-films and PCM-FP were delivered aboard the ISS by Northrop Grumman (NG-15) in Feb. 2021 for 6 months of exposure testing, including: temperature, vacuum, atomic oxygen, UV exposure and solar illumination effects. Our MISSE-14 PCM study will provide valuable information on the limitations and suitability of PCMs in harsh space environments.

Tunable and light-emitting metasurfaces have attracted increasing research interest in recent years. Here by combining liquid-crystal-integrated metasurfaces with a fluorescent substrate, we demonstrate active tuning of the emission spectrum and pattern in the red wavelength range. The measurements are performed with the techniques of momentum-space resolved spectroscopy and back-focal-plane imaging, showing a maximum of 16 nm shift of the emission wavelengths from 677 nm to 693 nm, and significant changes in the emission pattern at 660 nm. The results are further verified with numerical simulations. Our work paves the way towards actively controllable metasurface-based sources of complex light fields.

Knowledge of temperature-dependent optical properties of materials is required for photonics applications in extreme conditions, i.e., at high temperatures. In this talk, we will describe our latest measurements of temperature-dependent optical properties of materials (oxides, nitrides, semiconductors) for the development of metasurfaces for high-temperature applications that include thermal radiators and light sails. We use oscillator-based models to fit ellipsometry data at different temperatures in the wavelength region where a precise measurement can be made, and extrapolate to get broadband temperature-dependent optical properties. We also demonstrate using simulations how metasurface performance is affected by the temperature-dependence of constituent materials.

Metasurfaces have shown promise for the miniaturization of a wide variety of different optical components and have attracted significant interest for their ability to manipulate and control polarized light in a spatially-varying fashion. This capability, however, is not unique to metasurfaces. Several past technologies have been envisioned that spatially-varying control over polarized light. A scheme for classifying these is given, enabling rigorous comparison of the polarization control enabled by each. Moreover, Jones matrix holography, a concept which generalizes past design strategies for these elements, is introduced, and examples of new devices enabled by this generalized viewpoint are shown using metasurfaces.

Chirality is an intrinsic symmetry property of three-dimensional systems and arises when the system is distinguishable from its mirror image. New chiral systems capable of specifying and actively tuning circular birefringence would have broad implications in sensing, polarimetry, active quantum control, optical information processing, and communications. We present a reconfigurable chiral metasurface system that uses the shear displacement between two Pancharatnam-Berry metagratings to produce tailored broadband circular birefringence responses. Our dual metasurfaces give rise to a system that can break three-dimensional symmetry in order produce configurations ranging from achiral to chiral.

Three-dimensional elements, with refractive index distribution structured at subwavelength scale, provide an expansive optical design space that can be harnessed for demonstrating multifunctional free-space optical devices. We present three dimensional dielectric elements, designed to be placed on top of the pixels of image sensors that provide different functionalities like sorting and focusing of light based on its color, polarization and incidence angle. The devices are designed via iterative gradient-based optimization to account for multiple target functions while ensuring compatibility with existing nanofabrication processes. This approach combines arbitrary functions into a single compact element, even where there is no known equivalent in bulk optics, enabling novel integrated photonic applications. We analyze how the device behaves for input parameters that it was not designed for and investigate how the arrangement of the imaging pixels affects the device performance.

In electromagnetics, a medium moving at non-relativistic velocities is equivalent to a bianisotropic refractive index, produces the same effect for light as vector potential for charged particles. We investigate Mie scattering from the cylinder made of magneto-electric material. We find Poynting vector singularities in the near field distribution. A high k-vector region around the singularity may find applications in the near field superresolution imaging. We predict an additional phase change in the far-field for the magneto-electric cylinder compared to conventional Mie scattering. The magneto-electric coefficient can control the angular position of the phase change in the far-field and the position of the Poynting vector singularities.

The topological properties of the S-Matrix operating near their singular points offer new degree of freedom to address optical phase engineering. Here we engineer topologically-protected full 2𝜋− phase on a specific reflected polarization channel by choosing metasurface building blocks disposed along an arbitrarily closed trajectory in parameter space to encircle a singularity. The ease of implementation of the topological phase, together with its compatibility with other phase-addressing mechanisms including Pancharatnam-Berry phase, bring topological properties into the realm of industrial applications.

Topological insulators (TIs) are a new class of condensed matter system that host topologically protected surface states, leading to dissipationless electron transport. This intrinsic characteristic makes them potential candidate for quantum computing owing to their ability to preserve quantum coherence. Recently, these systems and the concept of topology have been embraced by the photonics community as well. In this work, we study the mid-infrared optical properties of high index (n~5.2) TI bismuth selenide (Bi2Se3) nanobeams (NBs), grown by chemical vapor deposition. Using Finite-difference time-domain (FDTD) simulations and FTIR nanospectroscopy, we find that these NBs support size-tunable Mie-resonant modes across the infrared (~1-16 µm). Furthermore, polarized measurements reveal that the total optical response of these deep subwavelength NBs is composed of TE and TM resonant mode. Finally, near-filed studies are also carried out to understand the effect of topological phase.

In this talk, I will discuss our recent efforts in the design, optimization, fabrication and characterization of all-dielectric metasurfaces enabling linear and nonlinear manipulation of the impinging optical wavefront using high-index contrast. During the talk, I will discuss the design principles, material needs, characterization results and opportunities for optical technologies.

By combining meta-optics and software backend, we can realize compact imaging systems with unprecedented functioncalities, including broadband aberration-free imaging, depth sensing and optical computing. We believe such hybrid digital-optical system will create a new research field on “Software Defined Optics”, akin to Software Defined Radio, where the software is used to simplify the hardware.

Optical metasurfaces offer unique opportunities for tailoring the interaction of light with nanoscale matter. Due to their flat nature, their integration with two-dimensional materials consisting of only a single molecular layer is particularly interesting. This talk reviews our recent and ongoing activities in hybridizing optical metasurfaces with different types of two-dimensional materials, including monolayer transition metal dichalcogenides (2D-TMDs). We demonstrate that metasurfaces enable careful control of the pattern and polarization of light emitted by the 2D-TMDs. Particular focus will be on the interaction of valley-polarized excitonic populations with various types of nanoresonators.

Birefringence is a fundamental optical property of anisotropic materials where the refractive index depends on the polarization of light, and is an essential property for devices such as waveplates and polarizers. In 2018, we reported barium titanium sulfide (BaTiS3) to have a broadband birefringence of 0.76 spanning the mid-to-far-infrared range, exhibiting the largest in-plane birefringence of any known bulk materials. In this talk, we will present the characterization of giant birefringence of two more engineered A1+xBX3 crystals, strontium titanium sulfide (Sr1+xTiS3) and barium titanium selenide (BaTiSe3). Our characterization combines polarization-resolved infrared spectroscopy with generalized ellipsometry to extract the optical properties.

Metasurfaces consisting of engineered nano-structures have shown exceptional abilities in light manipulation and have led to various practical applications. Using designed dielectric metasurfaces, we demonstrate spatial differentiation, a key aspect of optical analog signal processing, for broadband edge detection. Combined with quantum optics, the metasurface enabled spatial differentiator allows for edge detections with significantly higher signal to noise ratio compared to using classical optics. Furthermore, Fourier optical spin splitting microscopy based on a dielectric phase metasurface realizes single-shot quantitative phase gradient imaging. The proposed ideas pave the way for next generation high-speed real-time and multi-functional imaging.

Traditional refractive lenses are bulky owing to their curvature. Flat diffractive lenses can overcome this difficulty, but traditional diffractive optics have limited reach, primarily due to chromaticity. Recently, we have shown that by treating the “imaging” phenomenon as simply information transfer from the object to the image plane, the spatial distribution of the phase in the focal plane can be an arbitrary function. Using this concept, we have shown that allowing the phase in the image plane of a flat lens to be a free parameter enables imaging properties of unprecedented versatility in flat, multilevel diffractive lenses (MDLs). Our research group has demonstrated multi-level diffraction lenses in multiple high performance categories: unchromatic lenses with dramatically improved operating bandwidths, high NA and large aperture sizes, and extreme depth of focus.Furthermore, these can be combined with advanced machine-learning algorithms to enhance inferencing.

We present a novel metalens array for single-shot, large-volume, and high-resolution 3D imaging. Each metalens unit behaves as a special axicon to produce a Bessel beam point spread function (PSF), but with a bounded angular field of view (FOV). The metalens array can resolve three-dimensional information as a light-field camera. The bounded angular FOV prevents excessive image crosstalk between lens units to suppress background. Compared with conventional light-field cameras, our imaging device uses a single layer of metalens array without any bulk lens; the Bessel beam PSF increases the depth range that can be imaged while maintaining high resolution.

Efficient light manipulation at subwavelength scales in the mid-infrared (MIR) region is essential for various applications and can be harnessed from intrinsic low-loss dielectric resonators. Here, we demonstrate the fabrication of truncated spherical selenium (Se) resonators with tunable high-quality (Q) factor Mie resonances. Large area amorphous Se subwavelength resonators of varying sizes were grown on different substrates, using a novel CVD process. We demonstrate size-tunable Mie resonances spanning the 2-16 µm range, for single isolated resonators and large area ensembles, respectively. We show strong tunable absorption resonances (90%) in ensembles of resonators in a significantly broad MIR range. Moreover, by coupling resonators to epsilon-near-zero (ENZ) substrates, we engineer high-Q resonances as high as Q=40. We also show the resonance pinning effect near the substrate ENZ value, which is manifested in size-independent resonances.

Spectrally-tailored thermal emitters in the mid-infrared are needed for applications including gas sensing. We experimentally demonstrate a spectrally-selective, electrically-driven thermal emitter based on an aligned carbon nanotube metamaterial. Hyperbolic nanoribbon resonators patterned in the nanotube metamaterial double as resistive heaters and provide resonant polarized thermal emission in the infrared. The width of the nanoribbon resonators and their angle relative to the nanotube alignment axis can be designed to tailor the resonant frequency of the thermal radiation. Because of the low thermal mass of this design, the emitted thermal radiation can be modulated at rates up to 1 MHz.

We present a study of various compositions of the chalcogenide family used for static and active metasurfaces. We start with large area CVD grown amorphous spherical Selenium nanoparticles on various substrates and show that their Mie-resonant response spans the entire mid-infrared (MIR) range. By coupling Se Mie-resonators to ENZ substrates we demonstrate an order of magnitude increase in quality factor. Next, we investigate topological insulators Bi2Se3 and Bi2Te3 metasurfaces. We study the optical constants of single crystal Bi2Te3 in the NIR to the MIR range, followed by fabrication and characterization of metasurface disk arrays. We show that these high permittivity metasurfaces can yield very large absorption resonances using deep subwavelength structures. Finally, we demonstrate ultra-wide dynamic tuning of PbTe meta-atoms and metasurfaces, utilizing the anomalously large thermo-optic coefficient and high refractive index of this material.

With high feature density and subwavelength dimensions, visible spectrum metalenses are challenging to scalably manufacture. Electron beam lithography and short-wavelength photolithography capable of patterning metalenses for the visible do so at high cost per wafer. Here, we present a low-cost and scalable fabrication process based on nanoimprint lithography, and use it to demonstrate metalenses designed for 550 nm light with 4 mm diameter and NA=0.2. Our metalenses are formed of silicon nitride nanoposts with critical dimensions smaller than 100 nm. In this presentation we report focusing efficiencies above 50%, share holographic characterization data, and demonstrate imaging.

We describe a high throughput approach to all-inorganic metalens manufacturing using a single step nanoimprint lithography process and titania nanoparticle-based inks. The process yields a refractive index of 1.9, lenses with critical dimensions below 60 nm, feature aspect ratios greater than 8, and efficiencies greater than 55% and consistent device performance across 15 lenses printed within 30 minutes. We further describe pathways to fabricating all-inorganic lenses with RI of 2.1.

We present a technique for designing efficient and robust metasurfaces that optimizes the metasurface design curves instead of individual metasurface elements and thus does not suffer from the size limitations of conventional optimization techniques. Spatially varying design curves are parametrized and optimized using the grating averaging technique. We present simulation and experimental results of highly efficient metasurface beam deflectors and lenses that are robust to fabrication errors. In particular, we present an 80° beam deflector with absolute efficiency of 75% and a metalens with NA of 0.8 and an efficiency of 86% that is robust to fabrication errors.

When uncorrected, metalenses exhibit significant chromatic dispersion that limits them to narrowband operation. To address this, several metalens singlets corrected for chromatic aberration recently have been demonstrated. These metalenses mainly rely on engineered metaatom dispersion, an approach that limits them to small diameter and NA. Here, we validate an alternative approach by experimentally demonstrating a metalens doublet that directs light along trajectories of appropriate length to produce the desired achromatic behavior. Our lens is corrected over the 800-900 nm spectral region, collects light incident over a 2 sq. mm area, and has an NA of 0.2.

Over the year, lens designer have seen many new technologies growing up but metasurface is certainly a special of them. This talk will discuss how and why it is so fascinating for the lens design community. I will also describe how we can combine the power of modern optical design software and metasurface model to build new unprecedented tools to support raytracing through metasurfaces. Particularly, we will discuss about dispersion of high contrast metasurface for vis and NIR applications. It will also bring that a metasurface is far from a diffractive optical element in terms of dispersion engineering.

By combining state of the art optimization and machine learning techniques (photonics inverse design) with new fabrication approaches, we can implement large area metastructures and integrated photonics with superior photonics. In addition to making photonics more robust (e.g., to errors in fabrication and variation in temperature), more compact, and more efficient, this approach can also enable new functionalities. While in our early work we focused on inverse design and demonstration of individual photonic devices, our more recent work focused on scaling it to photonic integrated circuits and large area metasurfaces that can be fabricated in a commercial semiconductor foundry.

We present a technique for designing efficient and robust metasurfaces that optimizes the metasurface design curves instead of individual metasurface elements and thus does not suffer from the size limitations of conventional optimization techniques. Spatially varying design curves are parametrized and optimized using the grating averaging technique. We present simulation and experimental results of highly efficient metasurface beam deflectors and lenses that are robust to fabrication errors. In particular, we present an 80° beam deflector with absolute efficiency of 75% and a metalens with NA of 0.8 and an efficiency of 86% that is robust to fabrication errors.