Reciprocity condition in synchronously time-periodic bianisotropic materials

In this paper, a sufficient reciprocity condition for general time-periodic modulated bianisotropic media is extracted from first principles. Reciprocity of various cases of significant importance, including stationary bianisotropic media, time-varying (TV) isotropic media, TV anisotropic media, and TV bianisotropic media, are investigated using this condition. We prove that synchronous time modulation of stationary bianisotropic yet reciprocal media (chiral, pseudochiral, and achiral) does not lead to nonreciprocity, unless the modulation function breaks time reversal symmetry. This is in contrast to recently published research. The theoretical results are validated using in-house finite difference time domain simulations.