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

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.