Angular dependence of the reflection spectra and directional dispersion of the anisotropic exciton polaritons in ZnO

The reflection spectra of the ordinary and extraordinary polaritons near the A 1 ‐, B 1 ‐exciton resonances of ZnO are presented for two geometries: normal reflection of planes inclined to the c ‐axis at varying degrees, and oblique reflection of as‐grown (10 I 10) faces. The experimental and theoretical spectra are compared. The calculations are based on a) the dispersion relations derived from Fresnel's equation taking account of spatial dispersion, b) the Pekar additional boundary conditions combined with an exciton free surface layer. There are indications of the correlation between the thickness of this layer and decreasing oscillator strength of the polaritons.