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Phase-conjugation experiments have been performed in a backward degenerate four-wave mixing geometry using 1064 nm picosecond light pulses in thick p-toluene sulphonate polydiacetylene crystals. Two-photon absorption is identified and characterized. The relevant propagation equations are resolved analytically in different experimental geometries. Nonlinear absorption cancels out all the phase-conjugate signal at light fluences. A time-dependent analysis of the effect in a pump and nonlinear-probe experiment reveals an excited-state-induced nonlinearity.
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Nunzi et al. (1998) studied this question.
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