  In this paper, it is shown that non-reciprocity can be observed in
time-varying media without employing spatio-temporal modulated permittivities.
We show that by using only two one dimensional Fabry-Perot slabs with
time-periodic permittivities having quadrature phase difference, it is possible
to achieve considerable non-reciprocity in transmission at the incidence
frequency. To analyze such scenario,generalized transfer matrices are
introduced to find the wave amplitudes of all harmonics in all space. The
results are verified by in-house FDTD simulations. Moreover, in order to have a
simple model of such time-varying slab resonators, a time-perturbed coupled
mode theory is developed for multiple resonances, and it is shown that the
results obtained by this method and the analytical method are in excellent
agreement.
