Renormalization of polaritons due to virtual formation of biexcitons at high densities of excitation

The change of polariton dispersion is investigated theoretically if a large number of polaritons is present at frequencies near half the biexciton energy. A renormalization process takes place for polaritons via virtual formation of biexcitons and stimulated by the intense polariton distribution. Assuming an energetically sharp distribution as is realised e.g. by dye laser excitation the original polariton dispersion is shown to split into two new branches. These effects should manifest if e.g. an additional but weak probe light is used to testify the optical properties of the laser excited crystal. Furthermore the scattering lines of the two‐photon resonance Raman process via biexcitons are characteristically modified due to both selfrenormalization of the incoming polaritons and renormalization of the scattered polaritons. Explicit results are presented for CuCl where agreement with experimental data recently published is found.