Space-time Metasurface for self-adaptive retro-reflective Doppler Cloaks

Doppler cloaks are engineered electromagnetic covers, based on (space-)time modulated metamaterials and metasurfaces, able to compensate the Doppler effect induced by the motion of a scattered, making it appears as if it were at rest to a detecting radar system. Here, we propose the design of retro-reflective planar Doppler cloak composed by a pair of (space-) time modulated metasurfaces: the first metasurface focuses the incident field in a specific location on the second metasurface that is designed for enabling retro-reflection and Doppler frequency shift compensation. The proposed Doppler cloak is applied to a metallic planar reflector, moving towards its normal direction, and illuminated by an oblique plane wave, maintaining the radar cross-section of the reflector as much stable as possible within the designed angular range.