Jianji Yang

Large-Angle, Multifunctional Metagratings Based on Freeform Multimode Geometries

We show that silicon-based metagratings capable of large-angle, multifunctional performance can be realized using inverse freeform design. These devices consist of nonintuitive nanoscale patterns and support a large number of spatially overlapping optical modes per unit area. The quantity of modes, in combination with their optimized responses, provides the degrees of freedom required to produce high-efficiency devices. To demonstrate the power and versatility of our approach, we fabricate metagratings that can efficiently deflect light to 75° angles and multifunctional devices that can steer beams to different diffraction orders based on wavelength. A theoretical analysis of the Bloch modes supported by these devices elucidates the spatial mode profiles and coupling dynamics that make high-performance beam deflection possible. This approach represents a new paradigm in nano-optical mode engineering and utilizes different physics from the current state-of-the-art, which is based on the stitching of noninteracting waveguide structures. We envision that inverse design will enable new classes of high-performance photonic systems and new strategies toward the nanoscale control of light fields.

Freeform Metagratings Based on Complex Light Scattering Dynamics for Extreme, High Efficiency Beam Steering

Conventional phased-array metasurfaces utilize resonant nanoparticles or nanowaveguides to specify spatially-dependent amplitude and phase responses to light. In nearly all these implementations, subwavelength-scale elements are stitched together while minimizing coupling between adjacent elements. In this report, we theoretically analyze an alternate method of metasurface design, utilizing freeform inverse design methods, which support significantly enhanced efficiencies compared to conventional designs. Our design process optimizes wavelength-scale elements, which dramatically increases the design space for optical engineering. An in-depth coupled mode analysis of ultra-wide-angle beam deflectors and wavelength splitters shows that the extraordinary performance of our designs originates from the large number of propagating modes supported by the metagrating, in combination with complex multiple scattering dynamics exhibited by these modes. We also apply our coupled mode analysis to conventional nanowaveguide-based metasurfaces to understand and quantify the factors limiting the efficiencies of these devices. We envision that freeform metasurface design methods will open new avenues towards truly high-performance, multi-functional optics by utilizing strongly coupled nanophotonic modes and elements.

Analysis of material selection on dielectric metasurface performance

Dielectric metasurfaces are ultra-thin devices that can shape optical wavefronts with extreme control. While an assortment of materials possessing a wide range of dielectric constants have been proposed and implemented, the minimum dielectric contrast required for metasurfaces to achieve high-efficiency performance, for a given device function and feature size constraint, is unclear. In this Article, we examine the impact of dielectric material selection on metasurface efficiency at optical frequencies. As a model system, we design transmissive, single-layer periodic metasurfaces (i.e., metagratings) using topology optimization, and we sweep device thickness and light deflection angle for differing material types. We find that for modest deflection angles below 40 degrees, materials with relatively low dielectric constants near 4 can be used to produce metagratings with efficiencies over 80%. However, ultra-high-efficiency devices designed for large deflection angles and multiple functions require materials with high dielectric constants comparable to silicon. We also identify, for all materials, a minimum device thickness required for optimal metagrating performance that scales inversely with dielectric constant. Our work presents materials selection guidelines for high-performance metasurfaces operating at visible and infrared wavelengths.

High-performance axicon lenses based on high-contrast, multilayer gratings

Axicon lenses are versatile optical elements that can convert Gaussian beams to Bessel-like beams. In this letter, we demonstrate that axicons operating with high efficiencies and at large angles can be produced using high-contrast, multilayer gratings made from silicon. Efficient beam deflection of incident monochromatic light is enabled by higher-order optical modes in the silicon structure. Compared to diffractive devices made from low-contrast materials such as silicon dioxide, our multilayer devices have a relatively low spatial profile, reducing shadowing effects and enabling high efficiencies at large deflection angles. In addition, the feature sizes of these structures are relatively large, making the fabrication of near-infrared devices accessible with conventional optical lithography. Experimental lenses with deflection angles as large as 40° display field profiles that agree well with theory. Our concept can be used to design optical elements that produce higher-order Bessel-like beams, and the...

Silicon-based visible light meta-devices (Conference Presentation)

Semiconducting nanostructures are promising as components in high performance metasurfaces. We show that single crystal silicon can be used to realize efficient metasurface devices across the entire visible spectrum, ranging from 480 to 700 nanometers. Alternative forms of silicon, such as polycrystalline and amorphous silicon, suffer from higher absorption losses and do not yield efficient metasurfaces across this wavelength range. To demonstrate, we theoretically and experimentally characterize the resonant scattering peaks of individual single crystal silicon nanoridges. In addition, we design high efficiency meta-gratings and lenses based on nanoridge arrays, operating at visible wavelengths, using a stochastic optimization approach. We find that at wavelengths where single crystal silicon is effectively lossless, devices based on high aspect ratio nanostructures are optimal. These devices possess efficiencies similar to those made of titanium oxide, which is an established material for high efficiency visible wavelength metasurfaces. At blue wavelengths, where single crystal silicon exhibits absorption losses, optimal devices are instead based on coupled low aspect ratio resonant nanostructures and are able to provide reasonably high efficiencies. We envision that crystalline silicon metasurfaces will enable compact optical systems spanning the full visible spectrum.