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. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Automated summary

The study introduces a nonreciprocal EM isolating surface that generates OAM-carrying beams with electronic rotational Doppler effect using space-time-coding metasurfaces. Rotational Doppler shifts induced between two parallel OAM metasurfaces create electromagnetic wave isolation responses.