Polarization-independent multi-channel retroreflective metasurfaces based on extraordinary optical diffraction.

Retroreflection can be achieved by phase gradient imparted by super-cells of metasurfaces. Nevertheless, in most cases, retroreflection can only be achieved for one specific polarization. In this paper, we propose an alternative design strategy and reveal that a polarization-independent multi-channel metasurface based on extraordinary optical diffraction (EOD) can achieve high-efficient retroreflection. A unary unit cell, instead of binary unit cells, is employed to canalize impinging EM waves along targeted diffraction channels. Under oblique incidence, only the -1st diffraction order is maintained and the 0th order and others are suppressed through structural design while the reflection is unaffected under normal incidence. In this way, we can achieve retroreflection in three channels. A proof-of-principle prototype was designed, fabricated and measured to verify this design strategy. The prototype can operate at 20.0 GHz under the incident angle of ±48.6° and 0° with the efficiency of retroreflection about 90%. Both the simulated and measured results show an excellent performance of retroreflection along the three channels, regardless of the polarization state of incident waves. This method offers a fast implementation for retrodirective characteristics with facile planar fabrication and can also be easily extended to THz or optical regimes.