  Piezoelectric optomechanical platforms represent one of the most promising
routes towards achieving quantum transduction of photons between the microwave
and optical frequency domains. However, there are significant challenges to
achieving near-unity transduction efficiency. We discuss such factors in the
context of the two main approaches being pursued for high efficiency
transduction. The first approach uses one-dimensional nanobeam optomechanical
crystals excited by interdigitated transducers, and is characterized by large
single-photon optomechanical coupling strength, limited intracavity pump photon
population to avoid absorption-induced heating, and low phonon injection
efficiency from the transducer to the optomechanical cavity. The second
approach uses (quasi) bulk acoustic wave resonators integrated into photonic
Fabry-Perot cavity geometries, and is characterized by low single-photon
optomechanical coupling strength, high intracavity pump photon population
without significant heating, and high phonon injection efficiency. After
reviewing the current status of both approaches, we discuss the need for
co-designing the electromechanical and optomechanical sub-systems in order to
achieve high transduction efficiencies, taking the GaAs piezo-optomechanical
platform as an example.
