Résumé:

We report the observation of discrete modes in the upper branch of the exciton-polariton. In micrometer thick ZnSe epilayers grown on GaAs substrates by metal organic vapour phase epitaxy we have studied the evolution of the reflectivity spectra as a function of the thickness of the deposited ZnSe layers. For layer thickness in the range of a micron we observe reflectance oscillations that we interpret as due to Fabry-Perot modes in the upper branch of the polariton. This observation is possible due to the large exciton binding energy in ZnSe and the subsequent large energy extension of this branch. The amplitude of these oscillations and their energy splitting decrease with increasing thickness of the epilayer. They could no longer be observed for thickness larger than 4.5 mu m. The effect is analyzed in the context of a semiclassical approach using a non-local description of the dielectric constant, combined with results previously obtained in thin layers of bulk CdSe and CuCl.