Resonant nonlinear optical mixing using the phase conjugate configuration: Spectroscopic studies

We introduce the phase conjugate configuration for the study of ultrashort relaxation processes of the excited states of molecules having complex electronic spectra. This three beam configuration allowed us to distinguish between the optical and thermal portions of the four-wave nonlinear mixing in the frequency domain, and the typical characteristic features of the third-order nonlinear susceptibilities for all of the interesting difference frequency regions. The dispersive characteristics of the third-order nonlinear susceptibility, χ(3)(ωi = 2ω1−ω2) were interpreted by using a three-level model including spectral cross relaxation with which we discuss the various relaxation pathways and rates associated with the excited electronic states of cresyl violet and rhodamine 6G in ethanol.