A Spatio-Temporally Modulated Metasurface as a Free-Space N-Path System

A spatio-temporally modulated metasurface, that functions as a free-space N-path system, is reported at X-band frequencies. The reflection phase of the rows of the metasurface can be independently time-modulated for two orthogonal polarizations. A space-time bias is applied to the metasurface, enabling directionally dependent electromagnetic responses. When the modulation wavenumber is larger than that in free space, the metasurface suppresses certain harmonic mixing products in the far field, allowing subharmonic mixing. The metasurface was experimentally validated for a 2-path configuration, where the fabricated metasurface suppresses odd harmonic mixing products. With proper design of the space-time bias waveform, Doppler-like frequency translation is demonstrated at twice the modulation frequency.