RESONANT SCATTERING OF POLARITONS AS COMPOSITE PARTICLES

To calculate correctly the scattering of light by phonons or impurities in a crystal, the true asymptotic scattering states of the coupled system of crystal plus light (polaritons) should be used. When the light frequency is close to one exciton or optical-phonon frequency, the polariton is entirely excitonlike, and the polariton scattering can, in the Born approximation, be related to exciton scattering properties. If the exciton itself interacts strongly with an imperfection in the crystal, it is not permissible to treat either the exciton scattering or the exciton-photon interaction as perturbations. This problem of resonant scattering of polaritons is solved for short-range exciton-impurity interactions. Radiative damping and spatial dispersion appear in this solution in a natural fashion. Giant oscillator strengths of bound-exciton transitions are likewise automatically obtained. The proper inclusion of radiative damping and spatial dispersion keeps all cross sections finite. The relation between the theory and experiments is briefly discussed. © 1969 The American Physical Society.