Abstract
Some aspects of exciton polariton behaviour in anisotropic crystals are investigated for a specific model of the response function of an exciton split by a uniaxial field. The extraordinary-polariton dispersion scheme shows an additional branch for propagation in a nonprincipal direction appearing in the anomalous dispersion gap of the cubic case. The reflectivity spectrum obtained for this case is in good agreement with measurements performed on the basal plane of the uniaxial crystal PbI2 under oblique incidence. For a consideration of spatial-dispersion effects (finite exciton mass) the theory of Hopfield and Thomas is extended to uniaxial crystals. It is found that three simultaneous waves exist for every frequency and that a sharp extra reflection peak appears for every long-wavelength polariton mode of nonzero frequency. Reflection curves are calculated for different values of the exciton massm and the thicknesst of the Hopfield layer. The experimental normal-incidence «extraordinary” spectrum obtained by Biellmannet al. on the $$(10\bar 11)$$ of PbI2 which shows two additional sharp peaks can be reproduced by appropriate choice of the parametersm andt.
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