Coherent spectroscopy of collective excitations in superfluid helium far from equilibrium

TL;DR

This study uses ultrafast coherent control and time-resolved optical birefringence to explore nonequilibrium collective excitations in superfluid helium, revealing new insights into maxon pairs and Pitaevskii excitations.

Contribution

It introduces a novel ultrafast spectroscopy method to investigate multiple excitation branches in superfluid helium far from equilibrium, providing access to previously unmeasurable properties.

Findings

Maxon pairs exhibit surprisingly strong binding energy.

Maxon pairs have extremely short lifetimes.

Quasiparticle effective mass influences the phase of the coherent response.

Abstract

Ultrafast dynamics of collective excitations in superfluids remains largely unexplored beyond the roton regime, despite its importance for understanding nonequilibrium processes in these systems. Here, we employ ultrafast coherent control with sequences of femtosecond pulses to perform spectroscopy of multiple branches of the Landau excitation spectrum in superfluid helium far from equilibrium. By measuring the time-resolved optical birefringence, we track the nonequilibrium dynamics of maxon pairs and Pitaevskii plateau excitations alongside the previously studied roton pairs, revealing surprisingly strong binding energy of maxon pairs, their extremely short lifetime, and the influence of the quasiparticle effective mass on the phase of the coherent response. These results demonstrate the ability to extract previously inaccessible information about collective excitations in a strongly interacting quantum fluid by probing its nonequilibrium dynamics on femtosecond and picosecond timescales.