Propagation of exciton polaritons in inhomogeneous semiconductor films

INFM, Dipartimento di Fisica ‘‘A. Volta,’’ Universita`di Pavia, via Bassi 6, I-27100 Pavia, Italy~Received 28 July 1999!A theoretical study of the propagation of short light pulses in semiconductor slabs containing excitons hasbeen performed using the scattering-statetechnique and the steepest-descentmethod. These methods haveallowed us to calculate numerically and to describe analytically the temporary behavior of the dielectricpolarization inside the films as well as the time-resolved optical transmission spectra. The inhomogeneousbroadening of exciton resonances has been taken into account within a local model. The appearance of agrating of the dielectric polarization in a semiconductor film illuminated by a short light pulse is predicted.This grating, which is due to interfering exciton polaritons, moves backward with respect to the light-propagation direction. We have shown that reabsorption and re-emission of photons by excitons in quantumwells and semiconductor films have a crucial effect on the coherent time-resolved spectra of these structures.The time-resolved transmission decay rate is found to depend on the thickness of semiconductor films withinhomogeneously broadened exciton resonances, which is not the case for a purely homogeneously broadenedexciton. This fact, which follows from the energy dependence of the polariton damping in the former case, hasalso been explained in terms of multiple reabsorption–re-emission of photons by excitons.@S0163-1829~99!02548-5#I. INTRODUCTION