  We numerically study silicon waveguides on silica showing that it is possible
to simultaneously guide optical and acoustic waves in the technologically
important silicon on insulator (SOI) material system. Thin waveguides, or fins,
exhibit geometrically softened mechanical modes at gigahertz frequencies with
phase velocities below the Rayleigh velocity in glass, eliminating acoustic
radiation losses. We propose slot waveguides on glass with telecom optical
frequencies and strong radiation pressure forces resulting in Brillouin gains
on the order of 500 and 50,000 1/(Wm) for backward and forward Brillouin
scattering, respectively.
