High-resolution coherent probe spectroscopy of a polariton quantum fluid

TL;DR

This paper introduces a high-resolution coherent probe spectroscopy method to study elementary excitations in polariton quantum fluids, enabling direct observation of phononic and ghost modes and precursors to dynamical instabilities.

Contribution

The authors develop an original angle-resolved spectroscopy technique with unprecedented spectral and spatial resolution for studying polariton fluids.

Findings

Direct observation of low-energy phononic behaviour.

Detection of negative-energy ghost modes.

Identification of spectral features indicating dynamical instabilities.

Abstract

Characterising elementary excitations in quantum fluids is essential to study collective effects within. We present an original angle-resolved coherent probe spectroscopy technique to study the dispersion of these excitation modes in a fluid of polaritons under resonant pumping. Thanks to the unprecedented spectral and spatial resolution, we observe directly the low-energy phononic behaviour and detect the negative-energy modes, i.e. the \textit{ghost branch}, of the dispersion relation. In addition, we reveal narrow spectral features precursory of dynamical instabilities due to the intrinsic out-of-equilibrium nature of the system. This technique provides the missing tool for the quantitative study of quantum hydrodynamics in polariton fluids.