Study of exciton-polariton modes in nanocrystalline thin films of CuCl using reflectance spectroscopy

CuCl thin films grown on (100) Si by thermal evaporation are studied using reflectance spectroscopy. The reflectance spectra in the near UV spectral range close to the CuCl bandgap are modeled using a dielectric response function based on an exciton-polariton response with various models involving dead layers and reflected waves in the thin film. The exciton-polariton structure obtained is compared to other studies of bulk CuCl crystals. These different models are analyzed using a matrix-based approach and they yield theoretical spectra of reflected intensity. The fits provide parameter values which can be compared to bulk data known for CuCl and provide a non-destructive means of quantitative analysis of CuCl thin films. The best models are shown to match the experimental data quite well, with the closest fits produced when thin film front and rear interfaces are included. This model also accurately simulates the Fabry-Perot fringes present at energies lower than the Z₃ free exciton position in CuCl (at 3.272 eV).