Anti-reflection coating for all-semiconductor metasurface optical elements

Improving the performance of metasurface optical elements has become an increasingly important element of the ongoing quest toward their practical applications. One of the primary challenges is how to suppress light reflections across an entire metasurface. Such reflections are the source of undesirable noise, so their suppression is especially critical in imaging and optical communication applications. Here, we explore a variety of anti-reflection coatings (ARCs) for all-semiconductor transmissive metasurfaces and present a practical ARC that suppresses light reflection almost completely. Our numerical investigations reveal that the anti-reflection conditions of monolayer ARCs vary depending on the in-plane structural parameters of meta-atoms (circular posts or holes) as well as the plane on which an ARC is formed. We also found that such conditions can be well represented by our intuitive models. Furthermore, inspired by traditional ARCs for refractive optics, we investigated ARCs consisting of a bilayer as well. We found that an optimized bilayer ARC can significantly suppress reflections down to ∼0.5%, which is comparable to that obtained with traditional ARCs. We also demonstrate that creating nearly non-reflective metasurfaces is possible through the deposition of the bilayer ARC on an entire metasurface pattern. Given that the materials and configurations considered here are consistent with common manufacturing processes, this work can be a helpful guideline in the design of ARCs for metasurface optical elements.