  Coherent interconversion between microwave and optical frequencies can serve
as both classical and quantum interfaces for computing, communication, and
sensing. Here, we present a compact microwave-optical transducer based on
monolithic integration of piezoelectric actuators atop silicon nitride photonic
circuits. Such an actuator directly couples microwave signals to a
high-overtone bulk acoustic resonator defined by the suspended silica cladding
of the optical waveguide core, which leads to enhanced electromechanical and
optomechanical couplings. At room temperature, this triply resonant
piezo-optomechanical transducer achieves an off-chip photon number conversion
efficiency of -48 dB over a bandwidth of 25 MHz at an input pump power of 21
dBm. The approach is scalable in manufacturing and, unlike existing
electro-optic transducers, does not rely on superconducting resonators. As the
transduction process is bidirectional, we further demonstrate synthesis of
microwave pulses from a purely optical input. Combined with the capability of
leveraging multiple acoustic modes for transduction, the present platform
offers prospects for building frequency-multiplexed qubit interconnects and for
microwave photonics at large.
