  Surface acoustic waves (SAW) and associated SAW devices are ideal for
sensing, metrology, and connecting and controlling hybrid quantum devices.
While the advances demonstrated to date are largely based on electromechanical
coupling, a robust and customizable coherent optical coupling would unlock
mature and powerful cavity optomechanical control techniques and an efficient
optical pathway for long-distance quantum links. Here we demonstrate direct and
robust coherent optical coupling to surface acoustic wave cavities through a
Brillouin-like optomechanical interaction. In high-frequency SAW cavities
designed with curved metallic acoustic reflectors deposited on crystalline
substrates, SAW modes are efficiently optically accessed in piezo-active
directions that can be accessed through traditionally electromechanical
techniques as well as non-piezo-active directions that cannot. The non-contact
nature of the optical technique enables controlled analysis of dissipation
mechanisms and access to pristine mechanical resonators with record-level
quality factors (>100,000 measured here). The exceptional control of the
optical probe beams also enables detailed transverse spatial mode spectroscopy,
for the first time. These advantages combined with simple fabrication, small
size, large power handling, and strong coupling to quantum systems make SAW
optomechanical platforms particularly attractive for sensing, material science,
and hybrid quantum systems.
