Stacked Time‐Varying Metasurfaces

Abstract Spatiotemporal metasurfaces offer unique opportunities for wave manipulation; however, their practical realization is often constrained by the requirement for in‐plane spatial modulation, which necessitates a large number of time‐varying elements. In this work, an alternative architecture based on a cascade of spatially uniform metasurfaces subjected to periodic temporal modulation is introduced. Although all metasurfaces share the same modulation frequency, their individual modulation functions are independently engineered to achieve a desired complex electromagnetic response. A general theoretical framework is developed for the design and optimization of such stacked metasurface systems, composed of dense arrays of cylindrical meta‐atoms with time‐varying plasma and/or collision frequencies. The effectiveness of the approach is demonstrated through the optimization of metasurface designs that enable magnet‐free isolation at the fundamental frequency and a temporal analogue of circulators. Furthermore, it is shown that a metasurface stack can be implemented using only a few time‐modulated elements embedded within a parallel‐plate waveguide, opening new avenues for extremely compact, versatile, and scalable spatiotemporal platforms for next‐generation photonic and microwave systems.