Rotational Doppler effect by space-time-coding metasurfaces for nonreciprocal electromagnetic isolation.

Metasurfaces are artificial structures that can arbitrarily manipulate electromagnetic (EM) wavefronts. We propose a nonreciprocal EM isolating surface based on space-time-coding metasurfaces that generates orbital angular momentum (OAM)-carrying beams with electronic rotational Doppler effect. The region between two parallel 1-bit programmable space-time-coding OAM metasurfaces, one each for frequency and OAM order up-conversion and down-conversion, induce rotational Doppler shifts from opposing incident directions. An intermediate frequency-selective surface with highpass characteristics transmits the up-conversion signals and blocks the down-conversion signals. Hence, the EM waves are sensitive to illumination direction, exhibiting EM isolation responses, and the incident waves are only transmitted unidirectionally.