This conference presentation was prepared for SPIE OPTO, Photonics West, 2023.

Nonlocal metasurfaces have been enabling frequency-selective control over the spatial profile of light, but have thus far been limited to circular polarizations and/or 25% maximal efficiency. Here, we extend these capabilities to include any polarization state with near-unity efficiency. We report the finding of a polarization-agnostic geometric phase, and experimentally establish that it applies even to linear polarization states. Using symmetry-breaking design principles, this platform has complete control over all four physical degrees of freedom available to a Fano resonance (lifetime, resonant phase, and resonant polarization) and opens opportunities in augmented reality, secure optical communications, and nonlinear and quantum optics.

It will be shown that spectroscopic imaging ellipsometers (SIE) e.g. [1], can determine the number and size of inhomogeneity areas of smaller than 100 x 100 nm2 can be determined with an imaging ellipsometer on a field of view of e.g. a few µm by a few µm. In the contribution I will first discuss, how ellipsometric measurements with an imaging ellipsometer shall be interpreted 1) in the case of no depolarisation and with depolarisation. 2) in the presence of subwavelength defects or structures (Au) of an areal size of e.g. 50 x 50 nm and 10 nm height on Si can be easily detected by SIE, provided the two ellipsometric angles  and  are measured with an accuracy of 0.01°. This result proves that the well known sensitivity of ellipsometry on the thickness of vertical overlayers can be extended also to the horizontal dimensions[2,3]. [1] https://www.accurion.com/ [2] J. P. Perin and K. Hingerl, Appl. Surf. Sci., 421, 761 (2017) [3] K. Hingerl, Jour. Appl. Phys. 129, 113101 (2021)

Photonic bound states in the continuum (BICs) have enabled far-reaching nanophotonic applications in high-harmonic generation, biospectroscopy, and lasing, among others. BIC-based metasurfaces with tailored structural asymmetry have empowered these advances, but still face constraints related to large metasurface footprints, the need for complex polarization states, or fabrication limits requiring constant resonator heights throughout the structure. In this talk, I will present two directions for obtaining additional photonic functionalities in such systems: the arrangement of BIC-based unit cells in semi-infinite radial configurations for polarization invariance and reduced footprints as well as height-driven BICs leveraging resonators with different thicknesses as an additional degree of freedom for resonance engineering.

This conference presentation was prepared for SPIE OPTO, Photonics West, 2023.

The insensitivity of photons to magnetic fields calls for symmetry-based approaches in the design of photonic topological insulators. We experimentally demonstrate the realization of pseudomagnetic fields in deformed photonic crystals, and investigate the resulting photonic Landau-levels and guided topological states. Akin to strained graphene sheets, deformations (i.e., designed strain) in photonic crystals generate synthetic gauge fields for photons, leading to localization at flat bands and new types of topologically protected edge states at the boundaries of strained photonic crystals. We reveal that tailoring these gauge fields via strain-engineering yields new control over light dispersion, localization, and allows broadband minimization of radiation losses.

Modern imaging systems can be enhanced in efficiency, compactness, and range of applications through introduction of multilayer nanopatterned structures for manipulation of light based on its fundamental properties. Metaoptical components can be tailored to respond to these varying electromagnetic properties, but have been mostly explored in single-layer, ultrathin geometries, which limits their capacity for multifunctional behavior. Here we show the design of scattering structures which sort light efficiently based on its wavelength, polarization state, and spatial mode. The multispectral and polarimetry devices are further fabricated via two-photon lithography and experimentally validated in the mid-infrared.

We proposed the structural symmetry in a freeform color router design for subwavelength-pixel Bayer image sensors to insure the polarization independence as well as the symmetric field profiles. Although the reduced degrees-of-freedom lowers color routing efficiency compared to non-symmetric structures, it still outperforms the conventional color filtering approach. Numerical simulation confirmed that the proposed symmetric color router design for 0.5um-pixel Bayer sensor can achieve the peak optical efficiencies of ~0.8 for red and blue, ~0.7 for green regardless of the polarization direction (corresponds 3.2 times and 1.4 times enhancement over the ideal color filters). We believe that this study presents a more practical and efficient way to design color routers for CMOS image sensor applications.

Topology optimization has dramatically enhanced the design landscape of structures for realizing high-performance, freeform nanophotonic devices. However, unrestricted topology-optimized devices often possess complex geometries, making them difficult to fabricate in a reliable manner. Here, we impose hard geometric constraints by reparameterizing the design space. The design layouts are mathematically transformed from an unconstrained latent space to a physically realizable device composed of simple shapes. As a proof of concept, we apply the reparameterization technique to achieve high-efficiency large-angle wavefront transformation in periodic and aperiodic systems. We further expand the design framework to large-scale, multifunctional nanophotonic devices.

While conventional metasurfaces manipulate free-space light, here we overview two nonlocal metasurface architectures capable of customizing their own surface emission. The first, called “thermal metasurfaces”, produce thermal emission with custom wavefronts and polarization within the narrow bandwidth of a critically coupled quasi-bound state in the continuum. We report the first demonstration of unidirectional, spin-selective emission, achieved with silicon-based metasurfaces. The second, called “leaky-wave metasurfaces”, scatter guided waves to free space. These devices represent a generalized replacement for grating couplers, having equivalent form factor but complete control over the amplitude, phase, and polarization.

In nanophotonic, small mode volumes, narrow resonance linewidths and field enhancements, fundamentally scales with refractive index values and are key for many implementations involving light-matter interactions. Topological insulators (TI) are a class of insulating materials that host topologically protected surface states, some of which exhibit very high permittivity values. In this talk, I will discuss our latest results on Bi2Te3 and Bi2Se3 TI nanostructures. Using polarized far-field and near field nanospectroscopy we reveal that Bi2Se3 nanobeams exhibit mid-infrared resonant modes with 2π phase shifts across the resonance. We further demonstrate that Bi2Te3 metasurfaces exhibit deep subwavelength resonant modes utilizing their record high index value peaking at n~11.

Unidirectional transmission is a fundamental function in signal processing. In electronic systems, simple semiconductor p–n diodes provide one-way transmission for electrical signals. In the optical domain, however, achieving one-way transmission is difficult because it requires breaking the time-reversal symmetry of light–matter interaction. Previously, magneto-optic effects have been successfully utilized to break the reciprocity of light–matter interaction. Here, we propose a meta-optic diode that supports nonreciprocal light transmission through excitation of asymmetric quasi-bound waves. The meta-optic diode consists of an ultrathin dielectric slab, patterned with two types of subwavelength resonator arrays. The proposed approach is based on the lifetime engineering of the resonant modes in the arrays and the inherent Kerr nonlinearity of the dielectric slab.

Although photonic time crystals and time-varying metamaterials have been widely studied theoretically and numerically, experimental progress at optical frequencies has so far been limited. In this talk, we show that, despite the often-invoked space-time duality, the characteristics of the temporal dimension, particularly its relation to causality and energy conservation, can lead to various challenges and physical constraints that are unique to time-varying systems. Specifically, we discuss how temporal dispersion can lower the effective temporal modulation strength seen by the probe wave and we show that realizing large variations of the material permittivity in time may require large amounts of energy.

This work demonstrates the utility of a design, fabrication, and testing loop on 10 mm diameter metalenses to accelerate large-scale production of flat optics. By enabling rapid measurement and analysis of metalenses, it is possible to identify differences between designed performance and as-built performance quickly and correlate those to process characteristics. This accelerated feedback between the design, fabrication, and testing is expected to enable higher yields of better-performing metalenses.

Topology plays a fundamental role in contemporary physics and enables new information processing schemes and wave device physics with built-in robustness. Recently, significant efforts have been devoted to exploring topological principles in bosonic systems. In the first part of this talk, I will discuss my invention of topological lasers: integrable non-reciprocal coherent light sources as well as compact bound state in continuum sources. In the second part of the talk, I will discuss our recently proposed scheme to systematically implement singularities known as exceptional points (EP) in passive plasmonics. I will discuss the new scheme and how we overcame current immuno-assay nano sensing record with plasmons by more than two orders of magnitude. Finally, I will discuss how the topological phase around an EP singularity can be used to implement functional and multiplexed phased arrays, antennas, and metasurfaces.

The ideal imaging system would efficiently capture information about all fundamental properties light: intensity, direction, wavelength, and polarization. Most common imaging systems only map the spatial degrees of freedom of light onto a two-dimensional image sensor, with some wavelength and/or polarization discrimination added at the expense of efficiency. Thus, one of the most intriguing problems in optics is how to group and classify multiple degrees of freedom and map them on the two-dimensional sensor space. Here we demonstrate that volumetric meta-optics elements consisting of a highly scattering, inverse-designed medium structured with subwavelength resolution can sort light simultaneously based on direction, wavelength and polarization, by mapping these properties to a distinct combination of pixels on the image sensor for compressed sensing applications, including wavefront sensing, beam profiling, and next-generation plenoptic sensors.

Plasmonic resonance energy transfer (PRET) based imaging has been proposed as an effective label-free real-time biomolecular absorption spectroscopic method that enables the detection of biomolecules and dynamic cell activities. Here, we present a metasurface-based PRET-imaging platform that effectively engineers the darkfield scattering spectrum using the gap plasmon and grating effect. As a demonstration, three different biomolecules (Cyt c, Chl a, Chl b) are simultaneously detected using our metasurface-based PRET-imaging platform. Our proposed metasurface-based PRET-imaging platform enables continuous monitoring in the spatial region and multiplexed PRET sensing of multiple biomolecules with different absorption spectrums in the visible.

This conference presentation was prepared for SPIE OPTO, Photonics West, 2023.

We report the design, fabrication, and testing of electrically tunable metasurface k-space filters based on the phase-change material GSST and a transparent Si electrode heating architecture. A 10x10 array of PCM elements and Si heaters was fabricated via the foundry processes, and used to control the crystallinity of a metasurface consisting of GSST nano antennae. By selectively crystallizing individual elements, specific k-vectors of transmitted light can effectively be filtered out, resulting in improved imaging quality by reducing the scattered light that reaches the detector. Optimized doping profiles in the Si heaters allow for uniform, low power switching of the GSST state.

Integration of metasurfaces with planar waveguide integrated circuits can provide unique functionalities for beam shaping. Recent works show vertical integration of independently designed planar sources and metalenses, however this is not trivial or scalable. Here, we demonstrate an inverse design approach to co-optimize and merge grating and metalens functions into an out-of-plane beam shaper within a single 240nm silicon nitride layer. To exemplify, we demonstrate a 240nm thick meta-beam shaper that focuses 473nm light into a diffraction-limited spot with sidelobe suppression over 10dB. This approach for inverse design is generalizable to arbitrary beam shapes, unlocking a wide variety of applications.

In this presentation, I will show our recent progress on development of tunable dielectric metasurfaces with 1D and 2D arrays of individually addressable nanoantenna pixels. The former can be used for dynamic wide-angle beam steering applicable for LiDAR devices while the latter paves the way to future dynamic 3D holographic displays with wide field of view.

On-chip integration of metasurfaces with optoelectronic components enables the manufacture of reliable and low-cost systems that do not require post-fabrication alignment and assembly. Here we present an on-chip miniature beam scanning metasystem by integrating a large 2D array of VCSELs and a metalens. The beam scanner measures 4 mm*4 mm*3 mm and scans a ~1 mW laser beam at 940 nm with <1 degree divergence angle over a 140 degree*140 degree field of view with <30 mW of power consumption. The scanner switches between two angles in <1 µs, enabling the realization of fully-integrated miniature imaging lidar systems.

3-D sensing is a rapidly evolving technology impacting broad applications in computer vision. Existing 3-D cameras are restricted in field of view, resolution, and feature a complex multiple lens assembly. Here, we demonstrate a single piece fish-eye metalens in the visible band capable to perform diffraction-limited focusing and imaging over nearly 180-degree field of view. Our metalens has been shown to produce panoramic high-resolution imaging of a semi-circular target. Furthermore, we constructed a stereoscopic camera and performed panoramic depth measurements. We believe that the fisheye-metalens 3-D sensors is a promising platform empowering a range of emerging high-tech sectors.

Metasurfaces provide versatile platforms for arbitrary wavefront shaping with designer optical response such as amplitude, phase, and polarization at the deep subwavelength regime. Tunable metasurfaces can bring additional degree of freedom in terms of the time-dependent change of these responses, which can pave a way for novel applications such as wide-field-of-view holographic display and light detection and ranging (LiDAR). In this talk, we present the all-dielectric metasurface array that can modulate the phase of light above 270° in reflection with high reflectivity over 60% as a function of the individually applied voltage in the near infrared regime.

We demonstrate a novel paradigm for fiber sensing, based on the combination of an electrical sensor and a fiber-tip electro-optic metasurface. We transfer a semiconductor photodiode on the tip of an optical fiber and measure its current-voltage characteristics using an electro-optic photonic crystal. Any change in the electrical characteristics due to the environment can then be sensed via the fiber, greatly extending the application scope of fiber sensors. As a proof of principle, we demonstrate cryogenic temperature sensing.

The recent development of metasurfaces has enabled to make ultra-compact holographic displays and devices. In this talk, the design methodology, materials selection, multiplexing, dynamic modulation and conventional/practical fabrication for metasurfaces will be discussed in detail.

Sophisticated techniques to accurately align patterns have been developed that predominantly address the overlay of proximal layers in layer-by-layer processes that are separated by short, micron-scale distances. However, in some applications such as in flat optics, accurate, 3D alignment of workpieces whose patterns may be separated by large distances is desirable. Here, we describe an alignment scheme based on the far-field of a cascade of metasurface elements suitable for accurately aligning patterns separated by millimeter-scale distances. By using an 850nm laser illumination, we demonstrate registration accuracies of 10nm laterally and 50nm axially for patterns separated by mm-scale distances.

In nanophotonics, temperature variations shift the optical response through the thermo-optic (TO) effect (dn/dT) and may be detrimental in certain applications. We present a comprehensive solution to overcome the problem of optical constants thermal dispersion in nanophotonic and meta-optic devices. By using hybrid meta-atoms composed from two materials with positive and negative TO coefficients, we engineer metasurfaces with zero effective thermal dispersion (dneff/dT≈0). We demonstrate temperature independent resonant frequency, amplitude and phase response in metasurfaces, operating across a broad temperature range (ΔT=500K). Controlling the sign and magnitude of TO dispersion extends the capabilities of light manipulation in nanophotonic systems.

We present a novel spectro-polarimetric long-wave infrared (LWIR) thermal imaging system based on spinning metasurfaces. By utilizing the intrinsic dispersion in high-contrast anisotropic metasurfaces, we are able to generate tunable transmission spectra and achieve high spectral resolution. Our system enables simultaneous spectrum- and polarization-resolved thermal imaging, allowing for the extraction of hidden information in thermal radiation. Our low size, weight, and power (SWaP) system is a promising next-generation solution for high-quality thermal imagery data.