We demonstrate experimentally an unusual behavior of the parametric polariton
scattering in semiconductor microcavity under a strong cw resonant excitation.
The maximum of the scattered signal above the threshold of stimulated
parametric scattering does not shift along the microcavity lower polariton
branch with the change of pump detuning or angle of incidence but is stuck
around the normal direction. We show theoretically that such a behavior can be
modelled numerically by a system of Maxwell and nonlinear Schroedinger
equations for cavity polaritons and explained via the competition between the
bistability of a driven nonlinear MC polariton and the instabilities of
parametric polariton-polariton scattering.Comment: 5 pages, 4 Postscript figures; corrected typo