Cryogenic operation of MEMS-based suspended high overtone bulk acoustic wave resonators for microwave to optical signal transduction

Suspended high-overtone bulk acoustic wave resonators (HBARs) can serve as a viable optomechanical platform for efficient transduction of signals from the microwave to the optical frequency domain. In contrast to 1D nanobeam optomechanical crystals, HBARs benefit from very high RF to phonon injection efficiency ($\eta_{PIE}\approx$1) and low optical pump induced heating at cryogenic temperatures. By building small mode volume optical cavities around these devices, one can in principle achieve optomechanical cooperativities comparable to 1D nanobeam optomechanical crystals. In this work, we demonstrate cryogenic operation ($\approx$10 K) of such suspended HBAR devices and show classical signal modulation upto 3.5 GHz and response times $\approx$ 524 ns (for the fundamental mode at 340 MHz). While the transduction efficiency is currently limited by the material and device fabrication processes used in this work, we show that with reasonable modifications, efficient quantum transduction is within reach using this approach.