Micro-resonator optomechanics with superfluid helium

Abstract

Cavity optomechanics studies the interaction between light and the motion of mechanical oscil-lators, which can have mass ranging from cold atom clouds to kilogram-scale mirrors. By opti-mizing the optomechanical interaction and engineering the mechanical oscillators, researchershave generated macroscopic quantum states, and developed techniques for high precision sens-ing of mass, displacement and electric and magnetic fields. One of the most influential examplesof precision optomechanical sensing was the detection of gravitational waves. The techniqueof cavity optomechanics can also be leveraged to investigate superfluids. Owing to its zero vis-cosity, low optical absorption and high thermal conductivity, superfluid helium is an excellentcandidate for the mechanical oscillator of an optomechanical system. With the rich physics insuperfluid helium, the superfluid optomechanical system can be an ideal platform to study theinterdisciplinary field between quantum physics and condense matter physics. The general optomechanical interaction is mediated by the optical forces exerted on themechanical oscillator, which typically exist in the form of radiation pressure and photothermaleffects. In the first experiment reported in this thesis, a silica Whispering Gallery Mode (WGM)resonator is covered in a thin film of superfluid helium. Optical absorption in the WGMmicrotoroid resonator causes superfluid flow and evaporation, resulting in a recoil force thatexceeds the radiation pressure force by one order of magnitude. Using this superfluid-enhancedoptical force, we demonstrate feedback cooling of a bulk mechanical mode of the microtoroidwith final mode temperature as low as 137 mK and final mechanical occupancy of 2110 ± 40. In addition to using superfluid to control the motion of bulk modes of a microtoroid, wealso explore the native surface acoustic waves (third sound) that reside on the superfluid filmcondensed on the surface of the WGM resonator. Including the aforementioned advantages ofsuperfluid helium, the small mass and the self-assembling nature of the thin superfluid filmenable third sound to be a mechanical oscillator of high quality and large optomechanical cou-pling. We have theoretically developed a superfluid optomechanical system comprised of thirdsound waves and a WGM disk resonator, and estimated that the system is capable to achievean ultra-strong coupling, i.e. the single photon optomechanical rate larger than the mechanicalfrequency. Further, we theoretically and experimentally demonstrate that quantized vorticesin the thin superfluid film can interact with the third sound waves and lift the degeneracyof third sound modes. With the optical field and vortices bridged by third sound, the vortexdynamics can be probed using this superfluid optomechanical system. Experimentally, by mon-itoring the third sound frequency splitting we interrogate the coherent evolution of an ensembleof 17 vortices on the superfluid optomechanical resonator. The investigation of the coherentvortex dynamics in a nanometer-scale thin superfluid confined at microscale allows us to studythe fascinating physics in strongly-interacting superfluids, and build practical applications likehigh-precision inertial sensors. Finally a major work in this thesis is to study the Brillouin interaction between phase-matched travelling light and sound. The conventional Brillouin interaction is limited by thelarge Young’s modulus of solid materials, which constrains the electrostrictive compression ofthe material in response to light. The large mechanical compliance of the thin superfluid filmaffords us a strong Brillouin interaction between the travelling light and third sound confinedon a superfluid optomechanical disk resonator. Enabled by the strong Brillouin interactiona superfluid Brillouin laser is experimentally demonstrated to have the lowest so-far-recordedlasing threshold power of 1.8 µW, with harmonics of the Brillouin scattering observed up tothe 6th order at 5.6 µW. Furthermore, the combination of a strong Brillouin interaction andlarge Brillouin lasing amplitude allows the strength of the Brillouin-induced coupling betweencounter-propagating optical waves to exceed the optical damping rate for the first time. Thisstrong optical coupling has potential applications for all-optical reconfigurable optomechanicalcircuits and the generation of microwave frequency synthesis.