Cavity optomechanics focuses on the interaction between confined light and a mechanical degree of freedom. Vibrational modes of superﬂuid helium-4 have recently been identified as an attractive mechanical element for cavity optomechanics, thanks to their ultra-low dissipation arising from superfluid’s viscosity free flow. Our approach to superﬂuid optomechanics is based on nanometer thick films of superﬂuid helium which self-assemble on the surface of a microscale optical whispering gallery mode resonator inside our cryostat. Excitations within the ﬁlm, known as third sound, manifest as surface thickness waves with a restoring force provided by the van der Waals interaction. These excitations then dispersively couple to the light confined inside the optical resonator (see Fig. a). Using this optomechanical coupling mechanism, we experimentally probed the thermodynamics of these superfluid excitations in real-time, and demonstrated, for the first time, both laser cooling and amplification of the superfluid thermal motion. While lasers are widely used to cool gases and solid objects, they have never before been applied to cool a quantum liquid. These results have recently appeared in Nature Physics .
In addition we demonstrated an entirely new approach to optical forcing based on the atomic recoil of superfluid helium-4 (see Fig. b). This technique utilizes the thermomechanical effect of superfluids, whereby frictionless fluid flow is generated in response to a local heat source, which is the mechanism behind the famous superfluid fountain effect. This provides an order-of-magnitude boost in drive strength compared to conventional optical radiation pressure, which is important for applications ranging from microphotonic switches to fundamental quantum physics experiments. Furthermore, while the radiation pressure force exerted by a photon lasts for only the duration that the photon stays within the microphotonic element - typically less than a microsecond -, the superflow forces this photon can initiate can persist billions of times longer. This opens up unique new capabilities that could be used, for instance, to enable nonvolatile microphotonic memories and routers . Our research efforts are currently focused on developing new devices with greatly enhanced optomechanical performance (with single photon cooperativities C0 well in excess of unity), which will serve as probes of superfluid helium physics of unprecedented sensitivity , see Fig. (c). Our goals also include exploring the rich interaction between quantized vortices and third sound phonons and investigating thin-film superfluid inertial sensing directly on silicon chips. Recent results include: