We have calculated the two-photon absorption (TPA) spectrum in the Pariser-Parr-Pople model of conjugated polymers. Configuration interactions among all the single and double excitations are taken into account for relatively long chains. It is shown that the maximum of TPA decreases with increasing interaction strength or with increasing bond alternation. For moderate interaction strength and bond alternation, the overall TPA spectrum is characterized by two energy regions: the low-energy part due to mainly one-electron excitations (excitonic states), and the higher-energy part with contributions mainly from two-electron excited states (biexcitonic states). The high-energy TPA is about one order of magnitude stronger than the low-energy part. For stronger interaction or smaller bond alternation, the single and double excitations are fairly mixed, and the distinction between the excitonic and the biexcitonic regions is lost.

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Second Harmonic Generation on the Grating of Optical Inhomogeneities in ZnSe and ZnSNoncollinear second harmonic generation in ZnS and ZnSe crystals with quasi-regular grating of optical inhomogeneities is discovered and investigated. The observed effects are described by a model in which the vector phase synchronism may be realized in the region of normal crystal dispersion due to the contribution of the wave vector of the grating. 0 6~a p y w t e~ U HCCJIeAOBaH 3@$eKT HeKOJIJlHHeapHO% reHepaUHEi BTOpOfi IapMOHUKH B KpHCTWIJIaX ZnSe U ZnS CO&epXa~HX KBa3HpeIyJIRpHy€O peIIIeTKy OIITUYCCKHX HeOnHOpOnHOCTefi. Ha6nwnae~b1e 3aKOHOMePHOCTH 065XCHX€OTCX B03MOXHOCTbH) PeaJIHYdqUH 4a3OBOrO CUHXpOHU3Ma B o 6 n a c~n HOpMaJIbHOE ~CnepCUU KpHCTaJIJIa 38 CYeT BOJIHOBOIO BeKTOpa PeWeTKU. ') Leninskii Prospekt 70, SU-220602 Minsk, USSR. ') Partizanskii Prospekt 77, SU-220 600 Minsk, USSR.

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