Polaritons associated with nearly degenerate excitons: Their dispersion and the resulting reflectivity for the 1s exciton in CuBr

We propose a method to calculate the dispersion of polaritons associated with nonparabolic excitons. It gives directly the polariton wave vectors versus the photon energy, as required for a reflectivity calculation. For CuBr, we determine the normal reflectivity of the $1s$ exciton and find a good agreement with the experimental results. The existence of →k-linear terms explains the reflectance spike due to the triplet exciton. In most cases, an exciton state corresponds to a reflectivity structure. Sometimes, however, a given state is associated with none or with two singularities.