The field of optomechanics studies the interaction between electromagnetic and mechanical degrees of freedom via radiation pressure. This interaction is usually enhanced when both electromagnetic and mechanical degrees of freedom are normal modes of resonators, with the canonical optomechanical system being a cavity in which one mirror is mounted on a spring (thereby constituting a mechanical element). The majority of mechanical elements used in optomechanics to date are solid objects (mirrors, membranes, nanowires, etc); however fluids can also be used to form the mechanical element.
In this thesis, I describe the use of density waves in superfluid helium as a mechanical element in an optomechanical system. The reasons for using superfluid helium are the following: superfluid helium has high thermal conductivity, allowing it to be easily thermalized to cryogenic temperatures; it has negligible optical loss at IR wavelengths, which means it does not diminish the optical finesse of the cavity; finally its acoustic loss vanishes at low temperatures, allowing it to have a high mechanical quality factor.
In this system, we drive a normal mode of the density waves by modulating the optical intensity incident on the cavity. We also observe the mode’s undriven thermal motion and from that extract its phonon number. The optomechanical effects of optical spring and optical damping were observed, as were signatures of the mechanical mode’s quantum motion. These quantum signatures were the asymmetry between the Stokes and anti-Stokes sidebands, which arise from a combination of the mode’s zero point motion and the quantum backaction of the optical readout. We found agreement between these measurements and theoretical predictions (to within 20%) over a large range of mode temperatures.