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Planar Spin-Locked Retroreflector Made of Electric Metagrating with Near-Unity Efficiency

Metasurfaces based on subwavelength resonators have showing unprecedented ability of controlling the flow of electromagnetic waves, enabling many novel phenomena such as retroreflection. So far, the majority of retroreflectors focus on manipulation of linear polarized waves. Besides, the conversion efficiency of retroreflectors made of gradient metasurfaces is inherently limited. Here, we propose a metagrating based retroreflector for high-efficient spin-locked retroreflection and suppression of undesired diffractions. The handedness of the reflected waves is identical with the incident one. As a proof of concept, an electric metagrating based retroreflector has been theoretically designed at microwave frequencies and numerical results show a high-efficient retroreflection can be realized at incidence angle of 30°. The proposed retroreflector with ultrathin thickness is compact. Our design may serve as a promising platform towards spin-based retroreflection devices for not only interface electromagnetics but also ultra-flat photonics.