Time-Periodic Optical Activity

Utilizing coupled-wave theory, we investigate light propagation in a time-periodic bi-isotropic medium. Unlike its static counterpart, the dynamic medium couples co-handed counter-propagating waves, thus forming two momentum gaps in the Brillouin diagram for non-constant impedance. Chirality provides control over the resonance centers, bandwidths, and amplification factors in a distinct fashion for orthogonal polarization states. By deriving simple formulae for the scattering coefficients of a finite "time-slab" of the considered medium, we showcase how extreme optical rotation leads to a negative refraction regime. Even for weak chirality, whereby the chiral scattering coefficients reach a similar level to the corresponding achiral medium, we illustrate how elliptical polarizations can significantly change field orientation, a manifestation of temporal optical activity, while simultaneously being amplified.