A detailed theory is developed for the dephasing relaxation of the excitonic polariton which was measured recently by Masumoto, Shionoya, and Takagahara [Phys. Rev. Lett. 51, 923 (1983)] in samples of CuCl crystal by means of nondegenerate four-wave mixing. The concept of dephasing (transverse) relaxation of the excitonic polariton is clarified for the first time and the conceptual difference between the dephasing relaxation of the localized excitation and of the excitonic polariton is emphasized. A method of analysis of the experimental data of the nondegenerate four-wave mixing is presented in detail. The various mechanisms of dephasing relaxation of the excitonic polariton are discussed and examined quantitatively. It is found that the mutual collision among excitonic polaritons is the most probable mechanism that gives the correct order of magnitude of the dephasing relaxation rate and leads to reasonable energy dependence of the relaxation rate in agreement with the experimental results.
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