  Silicon photonics has offered a versatile platform for the recent development
of integrated optomechanical circuits. However, silicon is limited to
wavelengths above 1100 nm and does not allow device operation in the visible
spectrum range where low noise lasers are conveniently available. The narrow
band gap of silicon also makes silicon optomechanical devices susceptible to
strong two-photon absorption and free carrier absorption, which often introduce
strong thermal effect that limit the devices' stability and cooling
performance. Further, silicon also does not provide the desired lowest order
optical nonlinearity for interfacing with other active electrical components on
a chip. On the other hand, aluminum nitride (AlN) is a wideband semiconductor
widely used in micromechanical resonators due to its low mechanical loss and
high electromechanical coupling strength. Here we report the development of
AlN-on-silicon platform for low loss, wideband optical guiding, as well as its
use for achieving simultaneous high optical quality and mechanical quality
optomechanical devices. Exploiting AlN's inherent second order nonlinearity we
further demonstrate electro-optic modulation and efficient second-harmonic
generation in AlN photonic circuits. Our results suggest that low cost
AlN-on-silicon photonic circuits are excellent substitutes for CMOS-compatible
photonic circuits for building new functional optomechanical devices that are
free from carrier effects.
