Ultrafast non-equilibrium dynamics of rotons in superfluid helium

TL;DR

This paper introduces an ultrafast laser technique to probe non-equilibrium roton dynamics in superfluid helium, revealing rapid thermalization processes of roton pairs on a picosecond timescale.

Contribution

The study develops a novel ultrafast laser method to excite and measure non-equilibrium roton states in superfluid helium, advancing experimental capabilities in this field.

Findings

Ultrafast laser pulses can excite roton pairs in superfluid helium.

Roton pairs thermalize with the quasiparticle gas within picoseconds.

The technique enables real-time observation of non-equilibrium quasiparticle dynamics.

Abstract

Superfluid 4He, the first superfluid ever discovered, is in some ways the least well understood. Unlike 3He superfluid, or the variety of Bose-Einstein condensates of ultracold gases, superfluid 4He is a very dense liquid of strongly interacting quasiparticles. The theory is then necessarily phenomenological: the quasiparticle properties are found from experiment, and controversies over their description still remain, notably regarding vortex dynamics and the nature of rotons and roton pair creation. It is therefore important to develop new experimental tools for probing the system far from equilibrium. Here we describe a method for locally perturbing the density of superfluid helium through the excitation of roton pairs with ultrashort laser pulses. By measuring the time dependence of this perturbation, we track the non-equilibrium evolution of the two-roton states on a picosecond timescale. Our results reveal an ultrafast cooling of hot roton pairs as they thermalize with the colder gas of other quasiparticles. We anticipate that these findings, as well as future applications of the introduced ultrafast laser technique to different temperature and pressure regimes in bulk liquid 4He, will stimulate further experimental and theoretical investigations towards better understanding of superfluidity.