Transient phase conjugation in dye-doped polymer saturable absorbers

Dye-doped polymer saturable absorbers are an important class of nonlinear optical materials when nonlinear interactions like four-wave mixing and two-wave mixing can be observed even at low laser intensities. The modulation of the nonlinear absorption coefficient of the dye medium by an appropriate interaction of laser radiation can be utilized to generate a large phase conjugate (PC) signal in degenerate four-wave mixing geometries. By probing this population grating at time scales less than the dye relaxation times very high transient PC signals can be observed. Here we report on the generation of transient PC signals using azo dye-doped polymer films, by delaying the arrival time of the backward propagating pump beam with respect to the grating writing beams. A theoretical analysis of this delayed four-wave interaction is given to explain the large transient gain in the phase conjugate intensity. Experimental observations on the magnitude of the transient gain along with its dependence on various parameters like the intensity of the interacting beams, the number density of the saturable absorbers, the read beam time delay, etc. are also presented. (C) 1997 Elsevier Science B.V.