Brillouin resonance broadening due to structural variations in nanoscale waveguides

TL;DR

This paper investigates how structural variations in nanoscale waveguides cause resonance broadening in stimulated Brillouin scattering, revealing two mechanisms that influence the resonance width and shape, with implications for waveguide design.

Contribution

It identifies two distinct mechanisms of resonance broadening due to structural variations and explores their effects on SBS in nanoscale waveguides, including potential compensation strategies.

Findings

Structural variations cause homogeneous resonance broadening.

The second broadening mechanism is proportional to SBS gain.

Opposite sign broadening mechanisms can enable compensation.

Abstract

We study the impact of structural variations (that is slowly varying geometry aberrations and internal strain fields) on the width and shape of the stimulated Brillouin scattering (SBS) resonance in nanoscale waveguides. We find that they lead to an homogeneous resonance broadening through two distinct mechanisms: firstly, the acoustic frequency is directly influenced via mechanical nonlinearities; secondly, the optical wave numbers are influenced via the opto-mechanical nonlinearity leading to an additional acoustic frequency shift via the phase-matching condition. We find that this second mechanism is proportional to the opto-mechanical coupling and, hence, related to the SBS-gain itself. It is absent in intra-mode forward SBS, while it plays a significant role in backward scattering. In backward SBS increasing the opto-acoustic overlap beyond a threshold defined by the fabrication tolerances will therefore no longer yield the expected quadratic increase in overall Stokes amplification. Finally, we illustrate in a numerical example that in backward SBS and inter-mode forward SBS the existence of two broadening mechanisms with opposite sign also opens the possibility to compensate the effect of geometry-induced broadening. Our results can be transferred to other micro- and nano-structured waveguide geometries such as photonic crystal fibres.