Crossover from exciton to biexciton cavity polaritons

We report novel oscillator strength transfer phenomena induced by the excit on-exciton correlation. A high quality semiconductor microcavity is resonan tly excited by intense circularly polarised sigma(+) pump pulses (100 fs lo ng) and probed at different delays by weak counter-polarised sigma(-) test pulses. We show that the presence of sigma(+) excitons can completely inhib it the creation of sigma(-) excitons. With increasing sigma(+) exciton dens ity the oscillator strength of the a- transition is transferred to the biex citon transition, permitting the first observation of the strong coupling r egime between the biexciton transition and the cavity photon mode, i.e., th e biexciton polariton states.