All-Dielectric, Switchable Metalenses Using Sb2S3 Phase Change Material

All-dielectric metasurfaces have recently been shown to be excellent candidates for advanced wavefront manipulation devices in phase delays, beam steerers, metalenses, metaholograms and more. These devices allow for significant size, weight, and cost reduction in comparison to their traditional refractive optic counterparts, while allowing for creative use of non-linear responses of these structured surfaces. One limitation, however, for these devices is the lack of post fabrication tunability. To break this limitation, the fastest and most efficient approach would be utilizing refractive index-tunable materials. Phase change materials (PCMs) are one such category of materials. Huygens metasurface based optics utilizing these PCMs hold the possibility of high transmission and full 2π phase control of incident wavefronts which are tunable post-fabrication. In this work, we present a design and fabrication pathway for reconfigurable cylindrical metalenses in the visible utilizing one such PCM, antimony trisulfide (Sb2S3).<br/>PCMs aid in active optical modulation when switched between their different non-volatile phases. The selected PCM for this study, Sb2S3, exhibits a visible wavelength refractive index contrast of 0.75 with low losses when switched between its amorphous and crystalline phases (hereby called optical states). The metalens design presented here is comprised of an arrangement of pixelated arrays of homogenous nano-antennas. Each pixel is designed as an array of cylindrical nano-antennas in a repeating square lattice. Different geometric parameters are cycled through to generate a pool of Huygens’ meta-atoms to act as building blocks for individual pixels. We choose to discretize the continuous (ideal) 0 to 2π phase profile for a cylindrical lens into an arbitrary N = 4 phase levels. To create the discretized wavefronts necessary in both optical states, a total of N2 = 16 elements are chosen. This selection of elements informs the pixel sizes that are necessary for the wavefront-shaping. To address the inter-element coupling issues inherent in highly resonant Huygens metasurfaces, an inter-pixel gap of 1.5μm is employed, which effectively reduces the crosstalk between adjacent pixels.<br/>Individual nano-antenna arrays are optimized to ensure maximum generalized diffraction efficiency in both optical states of the PCM utilizing a figure of merit (FOM) first presented in [1]. This design FOM allows for a computationally effective and efficient synthesis of the reconfigurable metalens without the need for a full wave electromagnetic simulation of the entire metalens system in both optical states. The resulting compound metasurface acts as a reconfigurable cylindrical metalens with focal lengths of 3.5cm and 5cm depending on the optical state of the PCM The design predicts focusing efficiencies of 72.9% and 79.7% for the amorphous and crystalline phases, respectively.<br/>Sb2S3 thin films are created by ion sputtering with in situ substrate heating. The patterns are written using the Raith VOYAGER 100 electron-beam lithography tool on a Sb2S3 -PMMA- chromium stack and etched off in a single-etch process by reactive ion etching. This stack is then encapsulated with a layer of SiO2 to form a capping layer. The capping layer is necessary to prevent lateral migration of sulfur during the phase-change process, and to act as a mechanical protection layer for the metalenses. Optimization of nanofabrication, investigation of optically reversible phase changes, and experimental characterization of these metalens are all in progress.<br/>[1] M. Y. Shalaginov, "Reconfigurable all-dielectric metalens with diffraction-limited performance," Nature Communications, 2021.