Phase conjugation under two-photon absorption conditions in polydiacetylene crystals

Abstract

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.