Coherent spectroscopy of collective excitations in superfluid helium far from equilibrium
Ultrafast dynamics of collective excitations in superfluids remains largely unexplored beyond the roton region of the Landau excitation spectrum, despite the importance of such dynamics for understanding nonequilibrium processes in these systems. Here, we employ ultrafast coherent control with sequences of femtosecond pulses to perform spectroscopy of multiple quasiparticles in superfluid helium far from equilibrium. By measuring the time-resolved optical birefringence, we track the nonequilibrium dynamics of quasiparticle pairs associated with rotons, maxons and the Pitaevskii plateau region. The spectral lineshape of the roton peak is explained by an ab initio theoretical analysis of the roton-roton interaction. We also reveal strong energy shifts and short lifetimes of both maxon and Pitaevskii-plateau pairs, as well as an influence of the quasiparticle effective mass on the phase of their coherent response to laser pulses. 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 picosecond and sub-picosecond timescales.