Planar metasurface retroreflector

Metasurfaces are two-dimensional arrangements of subwavelength scatterers that control the propagation of optical waves(1-3). Here, we show that cascaded metasurfaces, each performing a predefined mathematical transformation(4), provide a new optical design framework(5) that enables new functionalities not yet demonstrated with single metasurfaces. Specifically, we demonstrate that retroreflection can be achieved with two vertically stacked planar metasurfaces, the first performing a spatial Fourier transform and its inverse, and the second imparting a spatially varying momentum to the Fourier transform of the incident light. Using this concept, we fabricate and test a planar monolithic near-infrared retroreflector composed of two layers of silicon nanoposts, which reflects light along its incident direction with a normal incidence efficiency of 78% and a large half-power field of view of 60 degrees. The metasurface retroreflector demonstrates the potential of cascaded metasurfaces for implementing novel high-performance components, and enables low-power and low-weight passive optical transmitters(6-8).