  We study the quantum state of phonons propagating on top of a fluid of light
coherently generated in a planar microcavity device by a quasi-resonant
incident laser beam. In the steady-state under a monochromatic pump, because of
the finite radiative lifetime of photons, a sizable incoherent population of
low frequency phonons is predicted to appear. Their mean occupation number
differs from a Planck distribution and is independent on the photon lifetime.
When the photon fluid is subjected to a sudden change of its parameters,
additional phonon pairs are created in the fluid with remarkable two-mode
squeezing and entanglement properties. Schemes to assess non-separability of
the phonon state from measurements of the correlation functions of the emitted
light are discussed.
