  The spectral and dispersive emission properties are analytically determined
for the two-dimensional system of exciton-polaritons in microcavities excited
by a resonant and coherent optical pump. New collective excitations result from
the anomalous coupling between one generic polariton state and its idler,
created by the scattering of two pumped polaritons. The corresponding
parametric correlation is stimulated by the emitter and idler populations and
drives very efficiently the luminescence. The intrinsic properties of the
collective excitations determine a peculiar emission pattern.
