Frequency-shifted reflection achieved through time-varying metasurfaces

In this contribution, we present a time-modulated full-reflective metasurface able to realize a frequency shift of a normally impinging electromagnetic plane wave. The frequency conversion is achieved by a dynamic control of the reflection phase, which emulates the phase advance (phase delay) of the field when reflected by a scatterer moving towards (away from) the source. The metasurface consists of three layers: the first two layers are used for realizing an array of mirrored mushrooms, i.e. two metallic patches printed on the two sides of a dielectric substrate and connected by a via; the third layer is a metallic ground plane that enforces the zero transmission. The patches are loaded by a set of varactors controlled by a low-frequency time-varying voltage signal. The metasurface and biasing network are properly designed for ensuring the operation for both polarization of the illuminating plane wave. We report here our results, showing the ability of the metasurface to reflect a frequency-shifted signal according to the modulation imparted by the varactors. The proposed metasurface can be used for realizing electrically thin Doppler cloak, which can restore the source illumination frequency of a moving object, as if it were not in motion.