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

Polaritons in a semiconductor microcavity excited below the free exciton en ergy have been studied by pump-and-probe experiments. The excitation result s in a blue shift of the transmission peak of the lower polariton branch, f ollowed by a strong enhancement and narrowing. These changes are opposite t o those caused by high above band gap excitation. The strong differences be tween resonant and non-resonant excitation result from a different impact o f localized excitons and unbound carriers on the quantum well susceptibilit y. The experimental observations are modeled in the approximation of nonper turbative coupling of the exciton system to the cavity light field. (C) 200 1 Elsevier Science Ltd. All rights reserved.