Optical Whispering Gallery Modes In Levitated Superfluid Helium Drops

Levitated superfluid Helium has a number of features that are well-suited to applications

in cavity optomechanics, including ultralow optical absorption, zero viscosity, high

thermal conductivity, and the ability to maintain sub-Kelvin temperature via evaporative

cooling. In addition, drops whose diameter approaches 1 mm should offer the possibility

of hosting high-finesse optical whispering gallery modes (WGMs). These modes are

expected to exhibit strong optomechanical effects, as the drop’s stiffness against

radiation pressure is provided only by the very weak surface tension of the superfluid.

Here we describe several new results from experiments with magnetically levitated

milimeter-scale superfluid 4He drops. Specifically, we have developed techniques that

allow us to trap a drop in ultra-high vacuum, to measure and control its size with

nanometer precision, and to suppress its center-of-mass motion to the nanometer scale.

We show that these drops reach evaporation rates well below 10^-12 m/s, corresponding

to temperature = 270 mK.

We find that free-space beams couple to a variety of WGMs with finesse up to ~ 1,000,

and we characterize these modes over a wide range of drop sizes. We find that the

WGMs’ frequencies, linewidths, and polarization dependence agree well with theory.

We will also describe ongoing efforts to couple to higher-finesse WGMs, and to

measure their optomechanical interactions with the drop’s surface waves.