Co-Prime Modulation for Space-Time-Coding Digital Metasurfaces with Ultralow-Scattering Characteristics

Multi-dimensional modulation of electromagnetic (EM) waves is critical in various fields such as electronic and photonic systems. Space-time-coding (STC) digital metasurfaces, an innovative platform for programmable metasurfaces, offer a straightforward and robust solution for spatio-temporal modulation, thereby enabling EM-wave manipulations in both space and frequency. Conventional STC schemes rely on periodic temporal modulations that are uniform in space (i.e., with same modulation frequency in all metasurface elements), and therefore do not take advantage of spatio-temporal coupling effects that can be induced by non-uniform periodic modulations (i.e., with different modulation frequencies across the metasurface elements). Here, a novel approach involving non-uniform spatio-temporal modulation, alongside co-prime modulation, is introduced. This approach enhances the fundamental principles of STC digital metasurfaces and facilitates the synthesis of angular scattering responses across different frequencies, guided by harmonic coupling conditions. The proposed strategy results in a more even distribution of the scattered intensity in both space and frequency. For validation, an experimental proof-of-principle for spatial-spectral diffuse scattering is presented. The experimental outcomes align closely with theoretical predictions, underscoring the effectiveness of co-prime modulation in achieving ultralow-scattering characteristics in STC digital metasurfaces. Moreover, this technique holds promise for applications in secure wireless communications, radar jamming, and complex beamforming. A novel approach involving non-uniform spatio-temporal modulation, alongside co-prime modulation, is introduced for space-time-coding digital metasurfaces. An experimental proof-of-principle for spatial-spectral diffuse scattering is presented at microwave frequencies. This approach enhances the fundamental principles of space-time-coding digital metasurfaces and facilitates the synthesis of angular scattering responses across different frequencies, guided by harmonic coupling conditions. image