Abstract

A microscopic theory is developed for time-domain and frequency-domain four-wave mixing of polyatomic molecules in condensed phases. The nonlinear response function R(t₃,t₂,t₁) is expressed in terms of the four-point correlation function of the dipole operator F(τ₁, τ₂, τ₃, τ₄) and is evaluated explicitly for a stochastic Langevin model of line broadening which holds for any correlation time of the solvent. Our results interpolate between the fast modulation limit, in which the optical Bloch equations are valid, and the static limit of inhomogeneous line broadening. Green function techniques are developed and used to perform the necessary summations over the vibronic manifolds required in the calculation of nonlinear susceptibilities in harmonic polyatomic molecules. Application is made to the calculation of excitation profiles of ground state CARS, excited state CARS, and incoherent Raman scattering of large polyatomic molecules in solution. The use of picosecond CARS as a probe for vibrational dephasing in liquids will be analyzed. In addition, a frequency-domain analogue of the transient grating experiment is developed. It is shown that degenerate four-wave mixing provides a sensitive probe for exciton dynamics (coherent, incoherent, and localized).

© 1986 Optical Society of America