Phase sensitive spectroscopy of the polariton modes in a ZnSe-ZnSxSe1−x heterostructure

Applying the method of spectral interferometry we investigate the phase of reflected light at a ZnSe-ZnS_xSe_1−x heterostructure. We find a series of polariton modes propagating through the ZnSe layer. They can be related to the different polariton branches split of at the heavy- and light-hole excitons. The phase shows pronounced changes around these modes. The strongest changes by 2π appear at the modes of lowest order located weakly above the exciton resonances, while they are smaller for higher modes. Our experimental findings can be explained considering spatial dispersion, Pekar's additional boundary conditions and a weak extension of the excitonic polarization into the ZnS_xSe_1−x cladding layers.