Goldstone mode in a quantum fluid of polaritons.

Goldstone modes appear as a consequence of a spontaneous breaking of a continuous symmetry. In driven-dissipative systems, they emerge in the long-wavelength limit of elementary excitations spectra in the form of overdamped modes whose linewidths tends to zero.

We study Goldstone modes physics in exciton-polariton quantum fluids which are a coherent superposition of cavity photons with quantum well excitons in planar semiconductor microcavities. In the optical parametric oscillation (OPO) regime of these systems, the crossing of the oscillation threshold, with the generation of signal and idler modes, constitutes a spontaneous symmetry breaking transition.

Using a new experimental method based on Bragg spectroscopy with a high spectral resolution, we present here the appearance of the Goldstone mode in the spectrum of elementary excitations of the polariton signal mode. Moreover, by fixing the phase of the signal mode by injecting an additional laser which breaks the initial U(1) symmetry, we observe the suppression of the Goldstone mode.