Theory of exciton-polariton generation by two-photon absorption

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Abstract
The simultaneous two-photon absorption and sum-frequency generation associated with exciton-polaritons in non-centrosymmetric crystals is treated theoretically. Expressions are derived for the two-photon absorption coefficient by solution of the coupled wave equations for the photons and the polariton, and by an alternative method based on linear response theory. The intensity of the generated polariton beam is calculated. The general theory applies to crystals of arbitrary symmetry properties, but particular attention is paid to the cubic case, appropriate to recent experiments on CuCl. The symmetry properties of the two-photon absorption are described for all possible polariton polarizations.