Tunable and Reversible Nonreciprocal Transmission with Cascaded Time-Modulated Metasurface

Nonreciprocal devices based on time modulation have garnered sustained interest in electromagnetic (EM) wave manipulation, due to their advantages of design flexibility, light weight, etc. However, existing time-modulated nonreciprocal metasurfaces are predominantly limited to modes operating between the fundamental and harmonic frequencies, with most of the energy diverted into harmonic channels, thereby limiting their application at the fundamental frequency. Here, a general cascaded time-modulated metasurface architecture is proposed that enables on-demand tunable and reversible nonreciprocal transmission at the fundamental frequency, with time-modulation frequencies far below the incident wave frequency. By controlling the harmonics interference through different time signals applied to each layer, the metasurface enables nonreciprocal wave propagation under either forward or backward incidence. Through theoretical analysis and full-wave simulations, a reconfigurable two-layer metasurface for nonreciprocal microwave transmission is ultimately designed and verified. Experimental results confirm that the nonreciprocal strength, propagation direction, and transmission peak frequency can be dynamically tuned by controlling the time-modulation signals. The method provides an approach for designing simple yet efficient and compact real-time tunable and reversible nonreciprocal metasurface devices, holding promising potential for applications in electromagnetic wave isolation, routing, and advanced signal processing etc.