  We propose an experimental setup to probe the low-lying excitation modes of a
parametrically oscillating planar cavity, in particular the soft Goldstone mode
which appears as a consequence of the spontaneously broken U(1) symmetry of
signal/idler phase rotations. Differently from the case of thermodynamical
equilibrium, the Goldstone mode of such a driven-dissipative system is an
overdamped mode, whose linewidth goes to zero in the long-wavelength limit.
When the phase of the signal/idler emission is pinned by an additional laser
beam in the vicinity of the signal emission, the U(1) symmetry is explicitely
broken and a gap opens in the Goldstone mode dispersion. This results in a
dramatic broadening of the response to the probe. Quantitative predictions are
given for the case of semiconductor planar cavities in the strong
exciton-photon coupling regime.
