Dissertation Defense: Charles Brown, Yale University, “Optical, Mechanical and Thermal Properties of Superfluid Liquid Helium Drops Magnetically-Levitated in Vacuum”

The field of optomechanics studies the interactions between light and the motion of an object. One of the goals in this field is to generate and control highly non-classical motion of a massive mechanical oscillator. There has been progress in generating such non-classical motion via coupling the oscillator to a qubit, or by utilizing the non-linearity of single photon detection. However, interest still remains in generating non-classical motion directly via the optomechanical interaction itself. Doing so requires strong coupling between the light and the mechanical oscillator, as well as low optical and mechanical loss and temperature. The unique properties of superfluid helium (zero viscosity, high structural and chemical purity and extremely low optical loss) addresses some of these requirements.
To exploit the unique properties of superfluid helium we have constructed an optomechanical system consisting entirely of a magnetically levitated drop of superfluid helium in vacuum. Magnetic levitation removes a source of mechanical loss associated with physically clamped oscillators. Levitation also allows the drop to cool itself efficiently via evaporation. The drop’s optical whispering gallery modes (WGMs) and its surface vibrations should couple to each other via the usual optomechanical interactions.
In this dissertation we demonstrate the stable magnetic levitation of superfluid helium drops in vacuum, and present measurements of the drops’ evaporation rates, temperatures, optical modes and surface vibrations. We found optical modes with finesse $\sim 40$ (limited by the drop’s size). We found surface vibrations with decay rates $\sim 1$ Hz (in rough agreement with theory). Lastly, we found that the drops reach a temperature $T\approx 330$ mK, and that a single drop can be trapped indefinitely.
Thesis Advisor: Jack Harris (jack.harris@yale.edu)