Symmetry-breaking due to material anisotropy for the stimulated-Brillouin-scattering opto-mechanical couplings in a nano-scale silicon waveguide: a numerical study based on the perturbation theory

Abstract Based on the perturbation theory, we conduct a numerical study on the opto-mechanical couplings of stimulated Brillouin scatterings (SBSs) in a nanoscale suspended rectangular silicon waveguide, in which the mechanical anisotropy of monocrystalline silicon is fully considered. First, it is observed that if a small angular misalignment of the order of 1° between the principal material axes and the symmetry axes of the waveguide geometry is introduced, the slight symmetry breaking can lead to the presence of some significant new resonant peaks in the SBS gain spectra. In particular, if a new resonant peak appears at the elastic frequency very close to that of a strong resonant peak already-existing in the spectrum, the observed phenomenon becomes much more pronounced, as an angular misalignment of just 1° can yield a new resonant peak with an SBS gain as high as 10 4 m −1 W −1 . To interpret the numerical results, a qualitative analysis based on the conventional non-degenerate perturbation theory is first provided. Thereafter, we further develop a rigorous perturbative model for close frequencies, which can accurately reproduce the more pronounced misalignment-sensitive phenomenon observed near strong resonant peaks in the spectra. Finally, the physical mechanisms underlying the numerically observed phenomenon are clearly revealed. The results obtained in the present study show that even minor errors in the crystallographic orientations introduced during the fabrication of a nanoscale silicon waveguide may have a non-negligible impact on its SBS opto-mechanical coupling characteristics. Meanwhile, the possibility of utilizing the strong mechanical anisotropy of silicon to realize potential sensitive harnessing and control purposes for practical SBS-based applications is also implied.