Bragg spectroscopy of a quantum fluid of polaritons

Exciton-polaritons in planar semiconductor microcavities, which can be considered as cavity photons dressed by interacting electron-hole pairs of quantum wells, are one of the most paradigmatic representatives of nonequilibrium quantum fluids of light in solid-state systems.

Like ultracold atomic condensates or liquid helium, polariton fluids support on their surface a set of collective excitations, the Bogoliubov modes, which are responsible for the emergence of macroscopic coherence phenomenons, such as superfluidity.

We show here a new experimental Bragg spectroscopy method allowing to exhaustively characterize the Bogoliubov relation dispersion of a coherently driven fluid of light in different regimes. The high spectral resolution of this technique allows us to accurately measure the speed of sound of polaritons and to observe the appearance of localized unstable modes. Finally, thanks to its high sensitivity, we can successfully reveal elusive spectral modes, such as the polariton ghost branch.