Depth-dependent eigenenergies and damping of excitonic polaritons near a semiconductor surface

This paper reports the experimentally observed change of exciton-polariton eigenenergies near the surface of a semiconductor. Normal-incidence-reflection spectra and attenuated-total-reflection (ATR) spectra are measured in the $n=1\mathrm{}$ exciton-polariton energy region. It is shown that ATR spectra probe regions near the surface whereas reflection spectra probe more deeply into the crystal bulk. Model calculations, which include spatial dispersion and depth-dependent eigenenergies and damping of excitonic polaritons, yield excellent agreement with experiments for various semiconductors. This agreement proves that reflection spectra are determined not only by bulk properties of excitonic polaritons, but reveal also the properties of the transition region at the crystal surface. Therefore, information extracted so far from reflection spectra about additional boundary conditions of excitons and exciton-free surface layers may have to be revised.