Stimulated inter-modal Brillouin scattering on suspended anti-resonant acoustic waveguides

Stimulated inter-modal Brillouin scattering (SIMS) is a versatile effect for implementing on-chip nonreciprocal isolators, frequency converters, and high-resolution filters enabled by the intrinsic decoupling of optical Stokes and anti-Stokes processes. However, the performance of integrated SIMS is typically hindered by weak spatial overlap between disparate optical and acoustic modes and the presence of multi-peak Brillouin gain spectra. In this work, we demonstrate inter-modal Brillouin scattering using a suspended anti-resonant acoustic waveguide (SARAW) architecture on a silicon-on-insulator (SOI) platform. By precisely tailoring the anti-resonance condition, we not only maximize the opto-acoustic interaction but also achieve high acoustic mode selectivity, effectively suppressing spurious mechanical resonances. Our device yields a single dominant Brillouin peak with a record-high inter-modal Brillouin gain coefficient of 718 W^-1m^-1 in silicon. This work opens new avenues for exploring advanced opto-acoustic interactions and optomechanical signal processing.