Angular-Adaptive Spin-Locked Retroreflector Based on Reconfigurable Magnetic Metagrating

Retroreflectors made of gradient metasurfaces have recently attracted intense interests due to their ability in reflecting incident light back to its source. So far, the current retroreflectors can only flip the transverse momenta of incident photons in specific incidence angles and thus have limitations in wide-angle applications. Here, a switchable metagrating based retroreflector is proposed for high-efficient spin-locked retroreflection and suppression of undesired diffractions. Upon reflection, the handedness of the waves is kept the same as the incidence. Furthermore, by mechanically altering the folding state of the reconfigurable retroreflector, adaptive tangential momenta could be imparted to the incidence photons, providing a high-performance retroreflection over a continuous range of incidence angles from 27.3 degrees to 52.5 degrees. As a proof of concept, a magnetic metagrating based retroreflector is fabricated at microwave frequencies and experimental measurements show consistent behaviors at various incidence angles. The proposed retroreflector is compact (overall thickness of 0.204 of the wavelength) and inherently insensitive to the illumination angle. As the design concept introduced in the paper could be extended to terahertz and optical frequencies, the design may serve as a promising platform toward reconfigurable spin-based retroreflection devices for not only interface electromagnetics but also ultraflat photonics.