High-Overtone Bulk Acoustic Resonators (HBAR) as cryogenic high-frequency Acousto-optic Modulators

The efficient transduction of signals from the microwave (3-8 GHz) to the optical frequency domain is critical for interfacing superconducting qubits with light. Resonant acousto-optics is a natural route for achieving high transduction efficiency as both the optical and acoustic field are enhanced in the same cavity geometry. Traditionally, nanoscale optomechanical cavities with piezoelectric transducers have been pursued as the geometry of choice for building quantum transducers [1] [2] . But, these devices have shown low transduction efficiency due to poor phonon injection efficiency from the transducer to the cavity mechanical mode. In this work, we take the opposite approach by engineering the acousto-optics interaction inside an HBAR, as reported in [3] , where the limited g 0 is compensated by increasing the cavity photon number ( N ), since the parametric interaction scales as g 0 $\sqrt N $ , providing an alternate route for efficient microwave-to-optical transduction at cryogenic temperatures. In addition, these cavity geometries are not affected by laser induced surface heating effects, which have severely limited the operation of nanoscale optomechanical cavities at cryogenic temperatures. The device comprise of a piezoelectric transducer (Aluminium Nitride) sandwiched between a top electrode (Aluminium) and a bottom electrode (doped Silicon), with thickness t Al = 1 µ m, t AlN = 0.5 µ m, t Si = 10 µ m, and t AU = 25 nm. The device operates as a High-overtone Bulk Acoustic wave Resonator (HBAR), where the resonant frequencies span from 300 MHz and 15 GHz, with modulation measured up to 5 GHz. In contrast to traditional AO interactions which are dominated by electrostriction, the optomechanical interaction here is dominated by the movement of the cavity boundaries, which is significantly enhanced in a suspended MEMS geometry. In this work, we demonstrate cryogenic operation of these devices as a first step towards engineering efficient quantum transducers in these micron-scale cavity geometries.