Polariton-polariton interaction constants in microcavities

Resonant transmission of light through a microcavity in the strong coupling regime is used to estimate the strength of the interaction between polaritons with parallel $({\ensuremath{\alpha}}_{1})$ or antiparallel $({\ensuremath{\alpha}}_{2})$ spins. The ratio ${\ensuremath{\alpha}}_{2}/{\ensuremath{\alpha}}_{1}$ is found to be strongly dependent on the detuning between exciton and photon modes. From negative to zero detuning it changes from about 0 to less than $\ensuremath{-}1$. Our observations indicate that at certain conditions the polaritons might rather condense in the real space than form a Bose-Einstein condensate. We analyze theoretically different mechanisms of polariton-polariton interaction including the mean-field electrostatic interaction, the direct exchange interaction, the Van-der-Waals interaction and the indirect exchange interaction via dark exciton and biexciton states.