EXCITONIC POLARITON INTERFERENCE IN THE REFLECTANCE OF GAAS THIN-FILMS

We have investigated the reflectivity of GaAs thin layers in the excit onic absorption region, observing detailed interference structures whi ch reveal the polariton dispersion and the quantization of the exciton centre-of-mass motion. To analyse the experimental spectra, we have c alculated the optical properties of thin films using a real-space dens ity-matrix approach adapted to the case of degenerate valence bands. T he oscillatory behaviour of the reflectance is well reproduced; a surf ace layer where the polarization decreases to zero is computed and its thickness is found to be energy dependent. The temperature dependence shows a broadening of the fine structure due to the phonon interactio n and a decoupling of excitons and photons at high temperature.