4D Wave transformations enabled by Space-Time metasurfaces: foundations and illustrative examples

Static metasurfaces have been shown to be prominent compact structures for reciprocal and frequency-invariant transformations of electromagnetic waves in space. However, incorporating temporal variation to static metasurfaces enables dynamic apparatuses that are capable of 4D tailoring of both the spatial and temporal characteristics of electromagnetic waves, leading to functionalities that are far beyond the capabilities of conventional static metasurfaces. This includes nonreciprocal full-duplex wave transmission, pure frequency conversion, parametric wave amplification, space-time (ST) decomposition, and ST wave diffraction. This article overviews our recent progress on the analysis and functionalities of ST metasurfaces and slabs to break reciprocity. We study different operation regimes of ST metasurfaces, such as scattering and diffraction at ST interfaces, ST sinusoidally varying surfaces and slabs, and transmissive and reflective ST metasurfaces. Additionally, corresponding applications are outlined.