Stimulated Brillouin scattering in a sub-wavelength anisotropic waveguide with slightly-misaligned material and structural axes: Misalignment-sensitive behaviors and underlying physics

The stimulated Brillouin scatterings (SBSs) in the sub-wavelength rutile waveguides with slightly misaligned material and structural axes are numerically studied. The misalignment is introduced between the extraordinary material axis and longitudinal axis of the waveguide only. Four nanowire waveguides with different cross-sectional geometries are considered. They consist of a circular waveguide, two elliptical waveguides with different cross-sectional orientation angles, and a trapezoidal waveguide with a completely unsymmetrical cross-sectional shape. As reported earlier, the resonant peaks emerging rapidly in response to the introduced small misalignment angle can also be observed in the calculated Brillouin gain spectra of the considered waveguides. But these misalignment-sensitive resonant peaks further exhibit some extraordinary behaviors, which may not be intuitively understandable. For instance, despite a plausible absence of symmetry breaking, many misalignment-sensitive resonant peaks can still be observed in the forward SBS gain spectrum of the trapezoidal waveguide. Based on the symmetry properties of the considered waveguides, the physics underlying the observed extraordinary phenomena are revealed. The obtained results highlight the effectiveness of introducing symmetry breakings for activating/harnessing opto-mechanical couplings in photonic-phononic micro structures, which would enable us to gain some deeper insights into the sub-wavelength opto-mechanics in anisotropic media.