Towards efficient microwave to optical signal transduction using confocal high overtone lateral bulk acoustic wave resonators (cXBARs)

Efficient microwave-to-optical transducers are essential for linking remote superconducting qubits using optical fibers. The vast majority of these devices use 1D optomechanical crystals for tight mode confinement and strong acousto-optic interaction but suffer from low phonon injection efficiency and parasitic surface heating at mK temperatures. In this work, we propose an alternative approach by using suspended high overtone lateral wave bulk acoustic wave resonators (XBARs). We fabricate prototype devices On an X-cut lithium niobate on an oxide platform and measure overtone resonances from 7-16 GHz. The lateral wave geometry allows one to incorporate small mode volume, high optical quality factor (Q<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</inf>) fiber cavities around the mechanical resonator— a unique feature of this design. Simulations show that efficient microwave-to-optical photon transduction is feasible in this geometry by exploiting the mechanical overtone at ≈ 12.02 GHz, matching the Brillouin frequency of silicon dioxide. Additionally, our work introduces confocal XBAR (cXBAR) to achieve higher mechanical Q<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> by confining the acoustic field and reducing energy leakage to the oxide substrate, as validated through simulation.