Dynamic polarization control by adjustable cross-polarization coupling in time-periodic anisotropic media

Abstract

We study wave propagation in an anisotropic medium whose optic axes vary periodically in time. This variation, in addition to generating frequency products, lifts the degeneracy between the two co-propagation modes, but unlike conventional time-periodic media, preserves the propagating nature of the modes. Coupled mode theory (CMT) relations show that the proposed modulation also equalizes the group velocity of cross-polarized pulses as in an isotropic medium. These properties make this structure appropriate for dynamic manipulation of propagating waves. For small variations in the direction of the optical axis, only the off-diagonal elements of the permittivity tensor are perturbed, which results in the generation of phase-adjustable cross-polarization oscillation (CPO). In addition to the ability to convert cross-polarized states, CPO offers a mechanism for wave retardation that stems from the dynamic phase-tunability of the mode profiles. This mechanism can provide a larger degree of freedom for retardation control. Furthermore, CMT relations near the CPO point of the proposed structure reveal its potential to produce desired mode profiles by dynamic manipulation, which can create a structure with adjustable sensitivity to circular polarization. This enables selective filtering of polarization states and offering a temporal circular dichroism. The analytical results are compared to numerical results from FDTD simulations.