  In the well-known stimulated Brillouin scattering (SBS) process, spontaneous
acoustic phonons in materials are stimulated by laser light and scatter the
latter into a Stokes sideband. SBS becomes more pronounced in optical fibers
and has been harnessed to amplify optical signals and even achieve lasing.
Exploitation of SBS has recently surged on integrated photonics platforms as
simultaneous confinement of photons and phonons in waveguides leads to
drastically enhanced interaction. Instead of being optically stimulated,
coherent phonons can also be electromechanically excited with very high
efficiency as has been exploited in radiofrequency acoustic filters. Here, we
demonstrate electromechanically excited Brillouin scattering in integrated
optomechanical waveguides made of piezoelectric material aluminum nitride
(AlN). Acoustic phonons of 16 GHz in frequency are excited with nanofabricated
electromechanical transducers to scatter counter-propagating photons in the
waveguide into a single anti-Stokes sideband. We show that phase-matching
conditions of Brillouin scattering can be tuned by varying both the optical
wavelength and the acoustic frequency to realize tunable single-sideband
modulation. Combining Brillouin scattering photonics with nanoelectromechanical
systems, our approach provides an efficient interface between microwave and
optical photons that will be important for microwave photonics and potentially
quantum transduction.
