Designing of strongly confined short-wave Brillouin phonons in silicon waveguide periodic lattices

TL;DR

This paper introduces a silicon waveguide design with a phononic crystal that enhances Stimulated Brillouin Scattering by confining phonons and reducing losses, enabling high gain and long-lived phonons for optomechanical applications.

Contribution

The paper presents a novel waveguide structure with a phononic crystal that significantly improves phonon confinement and reduces mechanical losses, advancing integrated Brillouin photonics.

Findings

Coupling factor of 0.54 (W.m)$^{-1}$ for backward scattering

Coupling factor of 4.5 (W.m)$^{-1}$ for forward scattering

30 dB attenuation of mechanical displacement after five unit cells

Abstract

We propose a feasible waveguide design optimized for harnessing Stimulated Brillouin Scattering with long-lived phonons. The design consists of a fully suspended ridge waveguide surrounded by a 1D phononic crystal that mitigates losses to the substrate while providing the needed homogeneity for the build-up of the optomechanical interaction. The coupling factor of these structures was calculated to be 0.54 (W.m)$^{-1}$ for intramodal backward Brillouin scattering with its fundamental TE-like mode and 4.5(W.m)$^{-1}$ for intramodal forward Brillouin scattering. The addition of the phononic crystal provides a 30 dB attenuation of the mechanical displacement after only five unitary cells, possibly leading to a regime where the acoustic losses are only limited by fabrication. As a result, the total Brillouin gain, which is proportional to the product of the coupling and acoustic quality factors, is nominally equal to the idealized fully suspended waveguide.