Polaritons de microcavité: Relaxation spontanée et stimulée

In an optical microcavity containing quantum wells, the strong coupling regime can be achieved between the optical and excitonic modes. The eigenstates of the system are mixed exciton-photon states, named cavity polaritons. The polariton dispersion is very different from the exciton dispersion. For large in-plane wave vectors, the polariton density of states is close to the exciton density of states, but in the center of the Brillouin zone, it can be 10 ³ or 10⁴ smaller. This property is promising to evidence a manifestation of the bosonic nature of polaritons and excitons. Thanks to the weak density of states in the Brillouin zone center, a large occupation factor of a partially excitonic state could be obtained. This large occupation could give rise to a stimulation the polariton relaxation by the final state population. In this experimental work, we study the relaxation of polaritons by performing photoluminescence and reflectivity measurements under various excitation conditions. Under non-resonant excitation, we evidence a bottleneck in the relaxation of polaritons, with an accumulation of polaritons in the large in-plane wave vector states. The relaxation bottleneck collapses when increasing the excitation density. By calibrating our experimental set-up, we show that polariton occupation factors close to one are never obtained in the strong coupling regime. The bottleneck collapse is not due to stimulated scattering but is due to the onset of polariton-polariton diffusion. On the contrary, using a second beam which resonantly creates a large occupation factor in a state of the Brillouin zone center, we evidence the stimulated scattering of polaritons which results simultaneously in a amplification of the resonant beam and a depopulation of the large in-plane wave vector states. These observations are a manifestation of the bosonic nature of polaritons.