Abstract
Temporally varying electromagnetic materials have recently attracted substantial attention as a platform for building tunable devices, being directly controlled by the developed waves. Such a concept can be expanded to metasurfaces, where the time modulation involves only the imposed boundary conditions without requiring the solving of a wave equation with time-dependent coefficients. In this framework, a generic class of two coupled sheets with temporally modulated surface conductivities is investigated under unilateral excitation. Various reflected and transmitted waves at different Floquet frequencies are determined by rigorously formulating the temporal dynamics and are found to give multiharmonic resonances. This spectral redistribution of power is adjustable via the time change of metasurface admittances, and thus can be used in a broad range of applications from secure transmission to dispersionless propagation.