  While nanoscale modal confinement radically enhances a variety of nonlinear
light-matter interactions within silicon waveguides, traveling-wave stimulated
Brillouin scattering nonlinearities have never been observed in silicon
nanophotonics. Through a new class of hybrid photonic-phononic waveguides, we
demonstrate tailorable traveling-wave forward stimulated Brillouin scattering
in nanophotonic silicon waveguides for the first time, yielding 3000 times
stronger forward SBS responses than any previous waveguide system. Simulations
reveal that a coherent combination of electrostrictive forces and radiation
pressures are responsible for greatly enhanced photon-phonon coupling at
nano-scales. Highly tailorable Brillouin nonlinearities are produced by
engineering the structure of a membrane-suspended waveguide to yield Brillouin
resonances from 1 to 18 GHz through high quality-factor (>1000) phonon modes.
Such wideband and tailorable stimulated Brillouin scattering in silicon
photonics could enable practical realization of on-chip slow-light devices,
RF-photonic filtering and sensing, and ultra-narrow-band laser sources by using
standard semiconductor fabrication and CMOS technologies.
