OPTICAL STUDIES IN ZNSE GAAS EPILAYERS - FABRY-PEROT MODES IN THE UPPER BRANCH OF THE POLARITON/

We report the observation of discrete modes in the upper branch of the exciton-polariton. In micrometer thick ZnSe epilayers grown on GaAs s ubstrates by metal organic vapour phase epitaxy we have studied the ev olution of the reflectivity spectra as a function of the thickness of the deposited ZnSe layers. For layer thickness in the range of a micro n we observe reflectance oscillations that we interpret as due to Fabr y-Perot modes in the upper branch of the polariton. This observation i s possible due to the large exciton binding energy in ZnSe and the sub sequent large energy extension of this branch. The amplitude of these oscillations and their energy splitting decrease with increasing thick ness of the epilayer. They could no longer be observed for thickness l arger than 4.5 mu m. The effect is analyzed in the context of a semicl assical approach using a non-local description of the dielectric const ant, combined with results previously obtained in thin layers of bulk CdSe and CuCl.