Some Evidence for the High Density Phase of Excitons in CdS

Reflection spectra without and with additional pumping are measured in CdS at 1.8 and 77 K, respectively, starting from very low pump intensities. At low pump intensities a special behaviour of the reflection minimum is found indicating the importance of an exciton dead layer. The experimental spectra up to the highest pump intensities can be fitted very well taking into account spatial dispersion, exciton dead layer, and density dependent excitonic parameters. Polarizability, damping, and layer thickness change continuously in the whole range of excitation intensities. On the other side, the transverse dielectric function of a high density gas of interacting Is excitons is calculated. A comparison between theoretically and experimentally obtained excitonic parameters at different densities shows good agreement. Hence, the importance of many‐exciton interaction for the reflection spectra up to vanishing at highest pump intensities is concluded.