Superfluid optomechanics

Superfluid optomechanics
An optical resonator covered by a thin superfluid film

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, by changing the amount of superfluid in the optical mode’s evanescent field, modulate the effective path length of the optical cavity. This provides a dispersive coupling between the superfluid and the light confined inside the optical resonator. 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 [Nature Physics, 12, 8, 788, 2016]. While lasers are widely used to cool gases and solid objects, they had never before been applied to cool a quantum liquid. Recent work includes exploring the rich interaction between quantized vortices and third sound phonons, enabling the first observation of vortex dynamics in a superfluid helium film [Science, 366, 1480, 2019]read more below, as well as the demonstration of ultra efficient Brillouin lasers based on superfluid helium filmsread more below.

Recent work:

Additional information on some of these publications can be found below:

Coherent Vortex Dynamics in a Strongly-Interacting Superfluid on a Silicon Chip

Artistic representation of a cluster of vortices orbiting a macroscopic circulation of opposite sign, Science, 366, p. 1480, Dec 2019
3D rendering file can be downloaded here.

Quantized vortices are central to the behavior of two-dimensional superfluids, as recognized by the 2016 Nobel Prize in Physics. They had however not been directly observed in superfluid helium films, due to the following experimental challenges:

  • the normal-fluid core of a vortex in superfluid helium-4 is roughly one Angström in size,
  • the thickness of a superfluid helium film is typically less than 20 nm, and
  • the refractive index of liquid helium is extremely close to that of vacuum (nHe=1.029).

Combined, these characteristics prevent direct optical imaging. In this work, we were able to generate and detect quantized vortices and observe their dynamics through an alternate mechanism, namely through their interaction with sound waves confined on the surface of an optical microresonator. The mechanism can be understood as follows. Waves (i.e. phonons) confined to the surface of the resonator (a silica microtoroid) can exist as either clockwise or counter-clockwise excitations.

These normally have the same energy/frequency and therefore appear as a single peak in our experimental spectra. In the presence of the background flow generated by a quantized vortex, however, these frequencies become different, which we can pick up experimentally, see e.g. [Physical review letters, 71, 1577, 1993]. (This is in essence, an acoustic analog of the Sagnac effect in optics, whereby the frequencies of third sound waves shift depending on the net circulation in the film). A third-sound wave traveling along the direction of circulation will be frequency upshifted, while a wave traveling against the vortex’s flow field will be frequency downshifted. Thus initially frequency-degenerate clockwise (cw) and counterclockwise (ccw) rotating third-sound modes will experience a frequency splitting due to the presence of a vortex. The precise strength of the interaction depends both on the sound mode and the spatial location of the vortex, and is discussed in detail in the following reference: [New Journal of Physics, 21, 053029, 2019].
With this understanding and using the techniques outlined in [Nature Physics, 12, 8, 788, 2016], we measure the vortex-induced splitting on several third-sound modes simultaneously. This is the key feature that enables us to discriminate between different vortex numbers and spatial distributions, as the vortices do not affect all third-sound modes identically.
This concept is analogous to sensing with a cantilever (see right image below), where the added mass due to the presence of a particle decreases the cantilever’s resonance frequency. Looking at a single vibrational mode, it is not possible to know whether the frequency change is due to a lighter particle located near the extremity of the cantilever, where its influence is maximal, or a heavier particle located near the cantilever’s anchoring point, where its influence is reduced. Tracking the effect on multiple modes simultaneously allows both particle mass and position to be independently determined, see e.g. [Nature Nano, 10, 339, 2015] and in our case vortex number and position, enabling effective imaging of the vortex distribution. Using this technique, we first employ a pulse of laser light to initiate two clusters of vortices of opposite sign and then nondestructively observe their decay in a single shot (see GP simulation by Dr Matt Reeves below). Thanks to the resonator’s near atomically flat surface, coherent dynamics dominate, with thermal vortex diffusion suppressed by five orders of magnitude.


Read more here:

Strong optical coupling through superfluid Brillouin lasing

Description coming soon…

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Keywords: Superfluid optomechanics, superfluid helium, Helium-4, quantized vortices, superfluid Brillouin laser, third sound, whispering gallery mode, microtoroid, laser cooling, vortex-sound interactions, superfluid thin films.

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