Breaking Reciprocity with Space‐Time‐Coding Digital Metasurfaces

TLDR

Metasurfaces are ultrathin engineered structures that can finely tailor electromagnetic wavefronts, and there is strong interest in using them to overcome Lorentz‑reciprocity limits with implications for communication, heat management, and energy harvesting, though experimental evidence has so far been limited to devices like circulators and antennas. The study leverages space‑time‑coding digital metasurfaces to break Lorentz reciprocity. The authors induce programmable spatiotemporal phase gradients via digital modulation of metasurface elements to produce anomalous reflections with frequency conversion, and they design and fabricate a microwave‑frequency prototype to validate the concept. The prototype demonstrates dynamically controllable nonreciprocal reflections that agree with theory, providing the first experimental evidence of space‑time‑modulated digital metasurfaces and opening possibilities for on‑demand applications such as frequency conversion, Doppler illusion, optical isolation, and unidirectional transmission.

Abstract

Metasurfaces are artificially engineered ultrathin structures that can finely tailor and control electromagnetic wavefronts. There is currently a strong interest in exploring their capability to lift some fundamental limitations dictated by Lorentz reciprocity, which have strong implications in communication, heat management, and energy harvesting. Time-varying approaches have emerged as attractive alternatives to conventional schemes relying on magnetic or nonlinear materials, but experimental evidence is currently limited to devices such as circulators and antennas. Here, the recently proposed concept of space-time-coding digital metasurfaces is leveraged to break reciprocity. Moreover, it is shown that such nonreciprocal effects can be controlled dynamically. This approach relies on inducing suitable spatiotemporal phase gradients in a programmable way via digital modulation of the metasurface-elements' phase repsonse, which enable anomalous reflections accompanied by frequency conversions. A prototype operating at microwave frequencies is designed and fabricated for proof-of-concept validation. Measured results are in good agreement with theory, hence providing the first experimental evidence of nonreciprocal reflection effects enabled by space-time-modulated digital metasurfaces. The proposed concept and platform set the stage for "on-demand" realization of nonreciprocal effects, in programmable or reconfigurable fashions, which may find several promising applications, including frequency conversion, Doppler frequency illusion, optical isolation, and unidirectional transmission.