Theory of high-density optical effects on polaritons in Cu$_2$O: Bistability and optical hysteresis
Oleksiy Roslyak and Joseph L. Birman

We present a comparative analysis of a 'conventional' phonoriton (coherent superposition of exciton-photon-phonon ) versus a polariton (coherent exciton-photon superposition) which is 'weakly' coupled to the LA-phonons bath. Depending on the duration of the pumping laser field the phonon-induced decoherence results in two distinct types of excitation. Long ($ms$) laser pumping pulses form an 'equilibrium' polariton. The generic feature here is a pronounced photo-thermal bi-stability. i.e. formation of four distinct branches. Transitions between branches can be achieved by excitation energy fluctuations as small as $200 \; neV$. This may impede BEC of the para-excitons. Short ($\mu s$) laser pulses create a 'quasi-equilibrium' polariton. In the latter case, for some critical intensity of the laser field we demonstrate the possibility of strong luminescence from a highly unstable localized state on the lower polariton branch.

© 2008 The American Physical Society.