  Phonons traveling in solid-state devices are emerging as a universal
excitation that can couple to different physical systems through mechanical
interaction. At microwave frequencies and in solid-state materials, phonons
have a similar wavelength to optical photons, enabling them to interact
efficiently with light and produce strong optomechanical effects that are
highly desirable for classical and quantum signal transduction between optical
and microwave. It becomes conceivable to build optomechanical integrated
circuits (OMIC) that guide both photons and phonons and interconnect discrete
photonic and phononic devices. Here, we demonstrate an OMIC including an
optomechanical ring resonator (OMR), in which infrared photons and GHz phonons
co-resonate to induce significantly enhanced interconversion. The OMIC is built
on a hybrid platform where wide bandgap semiconductor gallium phosphide (GaP)
is used as the waveguiding material and piezoelectric zinc oxide (ZnO) is used
for phonon generation. The OMR features photonic and phononic quality factors
of $>1\times10^5$ and $3.2\times10^3$, respectively, and resonantly enhances
the optomechanical conversion between photonic modes to achieve an internal
conversion efficiency $\eta_i=(2.1\pm0.1)%$ and a total device efficiency
$\eta_{tot}=0.57\times10^{-6}$ at a low acoustic pump power of 1.6 mW. The
efficient conversion in OMICs enables microwave-optical transduction for many
applications in quantum information processing and microwave photonics.
