Do bilayer metasurfaces behave as a stack of decoupled single-layer metasurfaces?

TL;DR

This paper investigates whether bilayer metasurfaces can be approximated as two decoupled single-layer metasurfaces, providing insights into their interaction and guiding efficient design strategies.

Contribution

The study evaluates the validity of treating bilayer metasurfaces as decoupled layers and offers a method to simplify their design process through targeted simulations.

Findings

Decoupled approximation holds in certain design regions.

Strong coupling regions require full-wave analysis.

Guidelines for efficient multilayer metasurface design.

Abstract

Flat optics or metasurfaces have opened new frontiers in wavefront shaping and its applications. Polarization optics is one prominent area which has greatly benefited from the shape-birefringence of metasurfaces. However, flat optics comprising a single layer of meta-atoms can only perform a subset of polarization transformations, constrained by a symmetric Jones matrix. This limitation can be tackled using metasurfaces composed of bilayer meta-atoms but exhausting all possible combinations of geometries to build a bilayer metasurface library is a very daunting task. Consequently, bilayer metasurfaces have been widely treated as a cascade (product) of two decoupled single-layer metasurfaces. Here, we test the validity of this assumption by considering a metasurface made of TiO2 on fused silica substrate at a design wavelength of 532 nm. We explore regions in the design space where the coupling between the top and bottom layers can be neglected, i.e., producing a far-field response which approximates that of two decoupled single-layer metasurfaces. We complement this picture with the near-field analysis to explore the underlying physics in regions where both layers are strongly coupled. Our analysis is general and it allows the designer to efficiently build a multi-layer metasurface, either in transmission or reflection, by only running one full-wave simulation for a single-layer metasurface.