Stimulated Brillouin scattering in a non-suspended ultra-low-loss thick-SOI platform

Silicon photonics, with its CMOS compatibility and high integration density, has enabled a wide range of novel applications. Harnessing stimulated Brillouin scattering (SBS), an optomechanic interaction between optical and gigahertz acoustic waves, in silicon-on-insulator (SOI) platforms attracts great interest for its potential in narrow-linewidth lasers and microwave photonics. However, the poor optoacoustic overlap in silicon nanowires on conventional SOI platforms has previously restricted the observation of SBS signals to suspended silicon waveguide structures. In this work, we report, for the first time, the SBS response in a non-suspended ultra-low-loss thick-SOI waveguide platform. The backward SBS process in this 3 μm thick SOI platform is enabled by a leaky acoustic mode that coexists with the optical mode in the waveguide core, resulting in an enhanced optoacoustic overlap. We measured Brillouin gain coefficients of 2.5 and 1.9 m⁻¹ W⁻¹ at 37.6 GHz for the rib and strip waveguides, respectively. This work paves the way for Brillouin-based applications in non-suspended ultra-low-loss silicon photonics systems.