Free-standing bilayer metasurfaces in the visible

TL;DR

This paper introduces free-standing bilayer metasurfaces made of TiO2 nanofins operating in the visible spectrum, enabling efficient wavefront shaping with high diffraction efficiency and overcoming limitations of previous multilayer designs.

Contribution

It presents a novel fabrication method for free-standing, bilayer metasurfaces with high efficiency, expanding capabilities in wavefront control and polarization manipulation in the visible range.

Findings

Achieved 80% diffraction efficiency in wavefront shaping

Developed a two-step lithography fabrication process for free-standing metasurfaces

Demonstrated polarization control using geometric phase in visible light

Abstract

Mult-layered meta-optics have enabled complex wavefront shaping beyond their single layer counterpart owing to the additional design variables afforded by each plane. For instance, complex amplitude modulation, generalized polarization transformations, and wide field of view are key attributes that fundamentally require multi-plane wavefront matching. Nevertheless, existing embodiments of bilayer metasurfaces have relied on configurations which suffer from Fresnel reflections, low mode confinement, or undesired resonances which compromise the intended response. Here, we introduce bilayer metasurfaces made of free-standing meta-atoms working in the visible spectrum. We demonstrate their use in wavefront shaping of linearly polarized light using pure geometric phase with diffraction efficiency of 80 % expanding previous literature on Pancharatnam-Berry phase metasurfaces which rely on circularly or elliptically polarized illumination. The fabrication relies on a two-step lithography and selective development processes which yield free standing, bilayer stacked metasurfaces, of 1200 nm total thickness. The metasurfaces comprise TiO2 nanofins with vertical side walls. Our work advances the nanofabrication of compound meta-optics and inspires new directions in wavefront shaping, metasurface integration, and polarization control.