Impact of exciton localization on the optical non-linearities of cavity polaritons

Polaritons in a semiconductor microcavity excited below the free exciton energy have been studied by pump-and-probe experiments. The excitation results in a blue shift of the transmission peak of the lower polariton branch, followed by a strong enhancement and narrowing. These changes are opposite to those caused by high above band gap excitation. The strong differences between resonant and non-resonant excitation result from a different impact of localized excitons and unbound carriers on the quantum well susceptibility. The experimental observations are modeled in the approximation of nonperturbative coupling of the exciton system to the cavity light field.