  Light-sound interactions have long been exploited in various acousto-optic
devices based on bulk crystalline materials. Conventionally these devices
operate in megahertz frequency range where the acoustic wavelength is much
longer than the optical wavelength and a long interaction length is required to
attain significant coupling. With nanoscale transducers, acoustic waves with
sub-optical wavelengths can now be excited to induce strong acousto-optic
coupling in nanophotonic devices. Here we demonstrate microwave frequency
surface acoustic wave transducers co-integrated with nanophotonic resonators on
piezoelectric aluminum nitride substrates. Acousto-optic modulation of the
resonance modes at above 10 GHz with the acoustic wavelength significantly
below the optical wavelength is achieved. The phase and modal matching
conditions in this scheme are investigated for efficient modulation. The new
acousto-optic platform can lead to novel optical devices based on nonlinear
Brillouin processes and provides a direct, wideband link between optical and
microwave photons for microwave photonics and quantum optomechanics.
