  We report on spinor polariton interactions in GaAs based microcavities. This
investigation is carried out by means of heterodyne polarized pump-probe
spectroscopy. We show the dependence of the energy renormalization of the lower
and upper polariton resonances with cavity detuning for different polariton
densities. We use the exciton-photon based Gross-Pitaevskii equation to model
the experiment for both lower and upper polariton modes. The theoretical
results reproduce qualitatively the experimental observations revealing the
magnitude and the sign of the parallel and anti-parallel spin interaction
strength. We evidence the strong influence of the biexciton resonance on the
anti-parallel spin polariton energy shift and provide the exciton-biexciton
coupling constant. We derive our results in the lower polariton basis using
Gross-Pitaevskii equation from which, we express analytically the spinor
polariton interactions and identify the clear role of the biexciton resonance.
