  Empowering independent control of optical and acoustic modes and enhancing
the photon-phonon interaction, integrated photonics boosts the advancements of
on-chip stimulated Brillouin scattering (SBS). However, achieving acoustic
waveguides with low loss, tailorability, and easy fabrication remains a
challenge. Here, inspired by the optical anti-resonance in hollow-core fibers,
we propose suspended anti-resonant acoustic waveguides (SARAWs) with superior
confinement and high selectivity of acoustic modes, supporting both forward and
backward SBS on chip. Furthermore, this structure streamlines the design and
fabrication processes. Leveraging the advantages of SARAWs, we have showcased a
series of record-breaking results for SBS within a compact footprint on the
silicon-on-insulator platform. For forward SBS, a centimeter-scale SARAW
supports a large net gain exceeding 6.4 dB. For backward SBS, we have observed
an unprecedented Brillouin frequency shift of 27.6 GHz and a mechanical quality
factor of up to 1,960 in silicon waveguides. This paradigm of acoustic
waveguide propels SBS into a new era, unlocking new opportunities in the fields
of optomechanics, phononic circuits, and hybrid quantum systems.
