Spatial-dispersion induced birefringence in cubic crystals: CuBr and CuI

A natural birefringence in single crystals of copper halides is observed and measured in the exciton part of the spectrum. This intrinsic birefringence is due to the spatial dispersion of the dielectric constant ϵ(ω, q ). Its spectral dependence and strong enhancement near the energy gap can be explained only, if we take account of the strong excitonic contribution in these compounds. This effect is related to the valence band anisotropy (warning) and thus allows a direct determination of the Luttinger band parameters | γ 2 - γ 3 | for these compounds.