Nonreciprocity in Bianisotropic Systems with Uniform Time Modulation.

Physical systems with material properties modulated in time provide versatile routes for designing magnetless nonreciprocal devices. Traditionally, nonreciprocity in such systems is achieved exploiting both temporal and spatial modulations, which inevitably requires a series of time-modulated elements distributed in space. In this paper, we introduce a concept of bianisotropic time-modulated systems capable of nonreciprocal wave propagation at the fundamental frequency and based on uniform, solely temporal material modulations. In the absence of temporal modulations, the considered bianisotropic systems are reciprocal. We theoretically explain the nonreciprocal effect by analyzing wave propagation in an unbounded bianisotropic time-modulated medium. The effect stems from temporal modulation of spatial dispersion effects which to date were not taken into account in previous studies based on the local-permittivity description. As a simple practical implementation, we design a deeply sub-wavelength bianisotropic metasurface which incorporates a single time-modulated capacitive layer backed by a usual dielectric layer. The designed nonreciprocal metasurface obeys generalized time-reversal symmetry, while it can exhibit strong unidirectional transmission or unidirectional amplification.