Vertical engineering for large stimulated Brillouin scattering in unreleased silicon-based waveguides

TL;DR

This paper introduces a vertical photonic-phononic engineering approach in silicon-based waveguides that significantly enhances stimulated Brillouin scattering without releasing the silicon core, simplifying fabrication and enabling large-area optomechanical devices.

Contribution

The study demonstrates a novel unreleased silicon waveguide design with a thick silicon nitride layer that reduces phonon leakage, achieving high Brillouin gain and advancing integrated optomechanics.

Findings

Achieved Brillouin gain around 300 (Wm)$^{-1}$ in unreleased waveguides.

Reduced phonon leakage by inserting silicon nitride layer.

Potential for observing Brillouin phenomena in large or compact devices.

Abstract

Strong acousto-optic interaction in silicon-based waveguides generally requires releasing of the silicon core to avoid mechanical leakage into the underlying silica substrate. This complicates fabrication, limits thermalization, reduces the mechanical robustness and hinders large area optomechanical devices on a single chip. Here, we overcome this limitation by employing vertical photonic-phononic engineering. Specifically, the insertion of a thick silicon nitride layer between the silicon guiding core and the silica substrate contributes to reduce GHz-frequencies phonon leakage enabling large values of the Brillouin gain in an unreleased platform. We get values of the Brillouin gain around 300 (Wm)$^{-1}$ for different configurations, which could be further increased by operation at cryogenic temperatures. These values should enable to observe Brillouin-related phenomena in cm-scale waveguides or in more compact ring resonators. This finding could pave the way towards large-area unreleased cavity and waveguide optomechanics on silicon chips.