In metamaterials engineering, there is currently a great surge of interest in exploiting temporal modulations of the constitutive parameters in addition to (or insted of) the conventional spatial one. In a series of ongoing studies, we have been exploring a class of “space-time” metasurfaces based on the concept of digital coding. These platforms rely on a limited number of (e.g., only two) inclusion types, whose “coding” description is particularly apt to the integration of active elements (e.g., diodes or micro-electro-mechanical systems) controlled by an integrated circuit, thereby leading to “programmable” metamaterial architectures.
In our proposed approach, via dynamic element switching in space and time, it is possible to perform simultaneous field manipulations in both space and frequency domains. For instance, the metasurface can re-radiate an impinging beam into multiple beams at different frequencies, with pre-designed directions and/or scattering patterns. Moreover, it is possible to apply the space-time-coding concept to break Lorentz reciprocity and realize free-space isolation, thereby enabling full-duplex communications. More recently, we have been exploring the simultaneous conversion of frequency and polarization via time-division-multiplexing metasurfaces, which should enable joint spatial/spectral/polarization field manipulations.
Within the overarching framework of “reconfigurable intelligent surfaces”, these platforms and concepts may find many potential applications in smart radio environments of interest for future wireless communication networks.