Controlling Nonreciprocity Using Enhanced Brillouin Scattering

The properties of space-time modulated media operating in the subsonic regime are discussed based on rigorous Bloch-Floquet theory. A geometrical description in the frequency-wavenumber plane is developed to provide insight into the possible interactions and their nature. It is shown that the secular equation has a singularity, which results in a weak/passive second harmonic generation process. In addition, bandgaps arising from the strong/active parametric interaction between an incident wave and its space-time harmonic result in an inelastic Brillouinlike scattering process. Hence, when the incident frequency is inside a forward (backward) bandgap, a Stokes' (anti-Stokes') scattered wave bounces back to the source. Although the forward and backward bandgaps do not generally occur at the same frequency bands, the insertion loss and gap width are equal. Requirement that both gaps do not overlap enforces a lower bound on the modulation speed. It is shown that although an increase in the modulation index is desirable, as it enhances the nonreciprocal behavior, it also limits the range of possible modulation speeds. It is shown that an alternative way to enhance the nonreciprocity, whenever the modulation index is constrained, is through the reduction of the modulation wavelength. The effective complex refractive index is calculated over a wide frequency range. It is shown that peaks appear in the extinction coefficient, indicating scattering to Stokes' and anti-Stokes' waves. Finally, a comprehensive numerical analysis based on the finite-difference time-domain method is developed to verify and demonstrate the intriguing properties of space-time modulated media.