Polariton Theory of Resonance Raman Scattering in Insulating Crystals

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Abstract
A multibranch polariton theory of Raman scattering in insulators is presented; actual calculations of the resonance cross section are carried out for an undamped two-branch model, for various values of photon-exciton coupling and background dielectric constant, including values appropriate to CdS. One obtains a double peaking with incoming photon frequency, corresponding to the experimentally observed in- and out-going photon resonances. Comparison with bare- and generalized-exciton computations shows that the results of the three theories are nearly the same for "smaller" couplings, as in CdS, except in the immediate region of the higher-energy peak; as the coupling increases from values near those in CdS, the predictions of the three theories begin to differ more substantially. When the exciton continuum is included by perturbation theory, the nearly similar predictions of the polariton and exciton theories, for frequencies below the bare-exciton frequency, are in agreement with experimental data on LO-phonon Raman scattering in CdS.