Space-time gradient metasurfaces

Metasurfaces characterized by a transverse gradient of local impedance have recently opened exciting directions for light manipulation at the subwavelength scale. Here we add a temporal gradient to the picture, showing that spatiotemporal variations over a surface may greatly extend the degree of wave manipulation in metasurfaces, and break several of their constraints associated with symmetries. As an example, we synthesize a nonreciprocal classical analog to electromagnetically induced transparency, opening a narrow window of one-way efficient transmission in an otherwise opaque surface. These properties pave the way to magnetic-free, planarized, nonreciprocal ultrathin surfaces for free-space isolation.