This PDF file contains the front matter associated with SPIE Proceedings Volume 12011, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.

We present conformal grayscale metamaterials that encodes multifunctional electromagnetic responses for a broad angle and broad bandwidth in an arbitrary form factor. Pushing the limit in the material degrees of freedom, the metamaterial assumes a high contrast, grayscale dielectric constants varying at the subwavelength scale. Conformal form factor is realized by discretizing the device into irregular voxels. Extremely high performing devices are inverse designed via gradient-based optimization and fabricated via additive manufacturing. We demonstrate a suite of multifunctional wave manipulations at RF frequency, including dispersion engineering, beam forming and steering in an airfoil-shaped structure, and conformal carpet cloak.

We report light trapping and guiding properties in resonant dielectric metastructures for chip-scale photonic integrated circuit applications. Recently, several optical phenomena in all-dielectric structures have shown a new way to tightly confine light through the engineering of resonant scattering. The engineered resonant light scattering in dielectric artificial subwavelength structures can be utilized as an efficient light coupling platform between free space and integrated photonic devices, and strongly tailor light-matter interactions for a variety of metasurface applications. In this paper, we present the design and numerical modeling of high-index subwavelength asymmetric resonant structures that can guide light for integrated photonic circuits. The metastructures reported here consist of all-dielectric two-dimensional optical antenna arrays patterned on a slab waveguide. Light coupling can be achieved by synchronizing the phase of the resonantly scattered light by the subwavelength antennas to that of the guided modes in the waveguide. Resonant light scattering by high-index subwavelength resonators can lead to light trapping through the excitation of quasi photonic bound states in the continuum. This unique feature can be used to selectively launch a guided mode into a photonic waveguide at a predetermined spectral band.

Resonant metasurfaces supporting quasi bound-states-in-continuum (BICs) resonances are particularly attractive for achieving high quality factor resonances which can be used to enhance nonlinear optical processes. In this work we first experimentally demonstrate quasi-BIC resonances in the mid infrared wavelength range using amorphous Germanium (a- Ge) based one-dimensional (1D) sub-wavelength grating structures with vertical asymmetry. The vertical asymmetric structures studied here consist of 1D a-Ge partially etched zero-contrast gratings (ZCG) on quartz substrate with the addition of a second asymmetric step-profile creating in-plane asymmetry. The vertical asymmetry added to the grating structure through the second etching step creates an open channel for accessing symmetry-protected guided mode resonances (GMR) which are otherwise experimentally inert to outgoing radiation. The fabricated device dimensions are: total height of 424 nm, ZCG etch depth of 190 nm, asymmetric step etch depth of 100 nm, pitch of 2.0 μm, and grating (asymmetric step) duty-cycles of 75% (32.5 %). FTIR measurements show clear resonance peak in the transmission spectra at ~3.2 μm wavelength for transverse magnetic (TM) polarized incidence with quality factor of ~50. We also characterize the incidence angle dependence of the measured resonance peak under classical and conical mounting condition and observe close to angle insensitive response for the latter. Next, we demonstrate third-order sum-frequency generation (TSFG) based up-conversion of the resonant mid infrared light to visible wavelengths in the presence of an additional 1040 nm pump excitation. Maximum TSFG enhancement of ~32 is obtained close to the mid infrared resonance.

Guided mode resonances (GMR) can be made angle insensitive using conical mounting of the grating relative to the external illumination. Conical mounting has been previously utilized for linear optical applications, such as optical filtering, and sensing. Here we present Third Harmonic Generation (THG) enhancement from 10 nm amorphous silicon overlayer on silicon nitride-based one-dimensional sub-wavelength grating GMR structures under conical-mounting illumination. The designed structure comprising of 70 nm deep silicon dioxide gratings of 1054 nm pitch, 50% duty cycle over which 160 nm thick silicon nitride and 10 nm a-Si layers are deposited is resonant at 1580 nm for TE polarized excitation. With increased angular spread of the incident excitation, the GMR spectral width and contrast are known to degrade. The angular aperture of the GMR structures studied here, which is defined as the angular spread across which the resonance drops to 50% of its peak value is calculated as 0.46° and 5.2° for classical and full-conical illumination respectively, highlighting the angular insensitivity of the full-conical mounting condition. Rectangular aperture masks placed in the back focal plane of the objective lens are used to limit the illumination angle along the grating wave-vector direction when compared to the grating line direction, thus achieving conical mounting condition. Experimentally, we observe the THG enhancement, defined as the ratio of on- to off-grating THG, improves from 2860 to 4742 and 1.7x10⁴ by utilizing 0.06 NA objective and illuminating in classical configuration (no aperture) with rectangular apertures of size 3x13 mm and 1.5x13 mm respectively.

Topological photonics attracts attention as a fundamental framework for robust manipulation of light. Combined with an optical gain, active topological cavities hold special promise for a design of high-performance nanolasers. In this talk, we present two types of novel topological resonant modes, multipolar lasing modes from topological corner states and ultralow-threshold lasing modes using super-bound states in the continuum, for the demonstration of low-threshold lasing.

Metasurfaces have been studied extensively in recent years as important platforms for controlling and guiding electromagnetic waves. In this paper, we introduce two metasurface designs that allow for controlling and steering different polarizations (spins) of surface waves. We also study a new type of surface waves that is supported by an L-shape design called chiral surface waves which are circularly polarized waves that possess two transverse spins. The spin direction is locked to the momentum of the surface wave which results in splitting it into two waves propagating in two different directions based on their helicity when excited with a linearly polarized source. This work provides a new way for manipulating the spin-orbit interaction of electromagnetic waves propagating along the surface. Controlling the spin-orbit interactions of electromagnetic waves is of great importance for applications in spintronics and valleytronics. Additionally, the surface modes supported by the presented metasurface designs have high self-collimation which can be used for polarization-based beam steering, spin filtering and surface wave guiding with the advantage of using homogeneous designs.

Wide field-of-view (FOV) functionality is crucial for implementation of advanced optical devices with applications spanning medical imaging, 3-D sensing, projection display, and security surveillance. While conventional wide FOV operation relies upon complicated assembly of multiple lens elements, metasurface optics offer a compelling alternative to realize compact, light-weight, and high-performance wide FOV optical modules. Here we elucidate the physical principles and design guidelines which underlies our recent demonstration of a fisheye metalens with > 170˚ diffraction-limited FOV. An analytical model is discussed, and wide-FOV achromatic metalens designs developed with a combination of direct search optimization and deep learning algorithms is presented.

The COVID-19 pandemic attributed to the SARs-Cov-2 virus has disrupted the lives of individuals in every corner of the world, causing millions of infections and numerous deaths worldwide. Identifying and isolating infected people is very crucial to slow down the spread of the disease. In this paper, we report a design of highly sensitive, graphene-metasurface based biosensor for detecting the S1 spike protein expressed on the surface of the SARSCoV-2 virus in the terahertz band. Our structure consists of a silicon dioxide substrate sandwiched between a complete gold layer at the bottom, and a graphene monolayer on top etched with a phi-shaped slot tilted at 45 degree, which performs a wideband reflective-type cross-polarization conversion of the incident electromagnetic (EM) wave. The optimized polarization conversion ratio (PCR) has been achieved at 0.75eV chemical potential value of the graphene layer. When samples of Sars-CoV-2 virus contained in a phosphate buffer saline (PBS) solvent is put on top of proposed design of the sensing surface, the spike proteins of the virus interact with the spike antibody grown on the sensing surface; and it changes the refractive index of the overall system (Biosensor + Analyte), which in turn changes the PCR and the corresponding frequency of the reflected wave. The biosensor response has been computed using the Finite Integration Technique (FIT) in the terahertz region. The sensitivity of the biosensor is found to be 354 GHz/RIU at the PCR of 0.9.

Metalenses are ultrathin optical devices designed to replicate behavior of conventional refractive lenses, or lens arrays, utilizing nanoscale resonant structures to redirect incident light. These are often comprised of discrete meta-atoms such as nanoscale dielectric pillars. Achromatic focusing—associated with traditional multi-element refractive counterparts—is frequently attempted with single-layer metalens designs, which has proven difficult to achieve with bounded refractive indices and total lens thickness. A recent study (F.Presutti and F.Monticone, 2020) formalized this, applying optical delay-line limitations to metalenses, resulting in a generalized trade-off in achromaticity for focusing systems.

In this work, we (1) theoretically explore achromaticity in metalens design, and (2) propose a thin-film multilayer design as an alternative to the discrete meta-atom approach for large numerical aperture (NA) achromatic metalenses. It is shown that wavefront modulation can also be achieved with spectrally-varying transmission magnitudes, rather than purely matching a phase profile. In fact, even with a bounded refractive index, perfect achromatic operation over a given spectral range can be offset by imperfect operation elsewhere, resulting in a NA limited by the smallest general spectral feature controlled. These considerations lead to a generalized phase-matching optimization routine, and a thin-film metalens is simulated, utilizing layered TiO2/MgF2 with total thicknesses ≤1 μm (≤20 layers), focusing across 6 simultaneous wavelengths (350-740 nm, Δλ~65 nm). A significant proportion (>40% spectral average) of the reflected light is focused for moderate NA (~0.35). With the maturity of the optical coating industry, the conformal thin-film approach reduces manufacturing complexity from its discrete nanoscale meta-atom equivalents.

We demonstrate the growth, assembly, and characterization of ultrahigh quality polaritonic systems based on α-MoO3 microplates and nanoribbons. These micro- and nanostructures are bottom-up-synthesized single crystals with minimal impurities. By optimizing the growth conditions, we also realize morphology control of the α-MoO3 structures. We observe highly confined polariton modes in the individual structures by using scattering-type scanning near-field optical microscopy. These highly confined polariton modes are of fundamental and technological interest.

We introduce WaveY-Net, a hybrid data- and physics-augmented convolutional neural network that can predict electromagnetic field distributions with ultra fast speeds and high accuracy for entire classes of dielectric photonic structures. This accuracy is achieved by training the neural network to learn only the magnetic near-field distributions of a system and to use a discrete formalism of Maxwell's equations in two ways: as physical constraints in the loss function and as a means to calculate the electric fields from the magnetic fields. As a model system, we construct a surrogate simulator for periodic silicon nanostructure arrays and show that the high speed simulator can be directly and effectively used in the local and global freeform optimization of metagratings.