Hydrodynamics at nanoscale: laser control of superfluid helium thin films
The importance of strongly interacting superfluids ranges from string theory to astrophysics; including dark matter, the quark-gluon plasma in the early universe, and the dense cores of neutron stars. However, progress is impeded across this full range of areas by the lack of a tractable underpinning microscopic theory. Superfluid helium is one of the only strongly interacting quantum fluids accessible on Earth, and therefore provides one of our few means for laboratory-based experiments towards extending our understanding. Exhibiting a macroscopic wavefunction, it also offers prospects for precision quantum sensing and computing technologies.
Two-dimensional films of superfluid helium exhibit complex turbulent dynamics, both due to hydrodynamic effect and because of the presence of quantized vortices. The details of these dynamics are a matter of wide debate. For instance, proposed values for the effective mass of a vortex range from zero to infinity, while soliton wave dynamics predicted may years ago have defied experimental observation. In this seminar I will provide an overview of recent work from my lab to develop new capabilities to address this debate.
We use optomechanical interactions in silicon-chip-based photonic devices to control and probe the hydrodynamics of thin superfluid helium films. This has allowed us to laser cool sound modes, generate and engineer gain processes that lead to sound lasing,[3,4] observe the coherent dynamics of quantized vortices, and study hydrodynamics at extreme length-scales.
This seminar will provide an overview of our work and then focus on recent unpublished work that confines superfluid helium-4 to a nanoscale photonic crystal cavity to create a miniature hydrodynamic wavetank. This provides both strong light-matter interactions and hydrodynamic nonlinearities that are five-orders-of-magnitude larger than has been achieved in any previous wavetank. It allows us to observe long-predicted phenomena, including backwards wave steepening and solitons. The solitons arise due to the balancing of dispersion and nonlinearity. They are dissipative in nature, with optomechanical gain processes compensating losses.
The ability to observe both vortex dynamics and hydrodynamic nonlinearities in superfluid helium films opens a new path to piece together accurate dynamic models and ultimately to build liquid quantum technologies. It also offers the possibility to study hydrodynamics in a regime that can only be accessible due to the zero-viscosity of superfluid helium – the viscosity of other liquids strongly suppresses wave dynamics in nanoscale films.
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 Harris et al, Nature Physics 12, 788-793 (2016); D. L. McAuslan, PRX 6, 021012 (2016).
 He et al, Nature Physics 16 417 (2020).
 Sawadsky et al, Science Advances 9 DOI:10.1126/sciadv.ade3591 (2023).
 Sachkou et al. Science 366 1480 (2019).