Flat Retroreflector Based on a Metasurface Doublet Enabling Reliable and Angle‐Tolerant Free‐Space Optical Link

Abstract Facilitated by the ability to reflect radiation along its incident direction, retroreflectors have been perceived as a pivotal component for establishing reliable free‐space optical links. However, conventional retroreflectors suffer from limited integration because of their bulky size, heavy weight, and nonplanar shape. Metasurface‐based devices consisting of subwavelength nanostructures combine semiconductor manufacturing methods with nanophotonics, regarded as a new platform that outperforms geometrical optics. In this paper, a free‐space optical link exploiting a flat retroreflector (FRR) based on metasurface doublet is proposed and realized at a telecommunications wavelength of 1550 nm. The top‐ and bottom‐layer metasurfaces, comprising hydrogenated amorphous silicon nanopillars based on a meticulously tailored dielectric spacer of silica, achieve the functions of a transmissive Fourier lens and a concave mirror, respectively. The top transmissive metasurface performs a spatial Fourier transform and its inverse, while the bottom reflective metasurface imposes a spatially varying momentum for reflecting beams along their incident direction. As a proof of concept, the designed FRR, precisely created via lithographical nanofabrication, has been readily applied to forge a substantially reliable free‐space optical link, featuring an enhanced angular tolerance of ±25°. This work will initiate a positive prospect for the cooperation between metasurface‐based devices and wireless optical communications.