Spatiotemporal modulation for non-reciprocal radiation and scattering

Lorentz reciprocity imposes constraints on radiation and scattering. For instance, a reciprocal antenna in a reciprocal medium will have identical radiation patterns in transmission and reception; therefore, in a dense environment a directive antenna will be prone to `hear' its own echoes. Likewise, a scattering matrix in reciprocal medium will be symmetric, implying particularly that a metasurface that reflects plane wave A to plane wave B will necessarily do the reciprocal process, from B to A, with identical efficiency. Recent works have shown that weak spatiotemporal modulation in high-Q ring resonators can create drastic non-reciprocity. Here we show theoretically and experimentally that, by proper design, taking advantage of the light-cone filtering property, spatiotemporal modulation of low-Q radiating and scattering systems, can exhibit also significant non-reciprocity. Possible applications are in radio-frequency communication systems, as well as open new directions in nanophotonics, energy harvesting, and thermal management, when translated to different frequency regimes.