  Cavity optomechanics allows the parametric coupling of phonon- and
photon-modes in microresonators and is presently investigated in a broad
variety of solid-state systems. Optomechanics with superfluids has been
proposed as a path towards ultra-low optical- and mechanical-dissipation.
However, there have been no optomechanics experiments reported with non-solid
phases of matter. Direct liquid immersion of optomechanics experiments is
challenging since the acoustic energy simply leaks out to the higher-impedance
liquid surrounding the device. Conversely, here we confine liquids inside
hollow resonators thereby enabling optical excitation of mechanical
whispering-gallery modes at frequencies ranging from 2 MHz to 11,000 MHz (for
example, with mechanical Q = 4700 at 99 MHz). Vibrations are sustained even
when we increase the fluid viscosity to be higher than that of blood. Our
device enables optomechanical investigation with liquids, while light is
conventionally coupled from the outer dry side of the capillary, and liquids
are provided by means of a standard microfluidic inlet.
