Heterodyne-detected stimulated photon echo: applications to optical dynamics in solution

Heterodyne detection of the stimulated photon echo (HSPE) is discussed and applied to explore molecular solvation dynamics. With this technique the in-phase and in-quadrature parts of the induced transient nonlinear polarization can be time-gated. A third-order perturbative description of the HSPE is presented, which shows that the transient polarization provides direct information on the real and imaginary components of the nonlinear optical response functions of the system. In the spectral diffusion limit and for impulsive excitation conditions, analytical expressions for the HSPE signal are derived and model calculations are presented. This novel technique is applied to a nonlinear optical study of the dye molecule DTTCI dissolved in ethylene glycol. Several effects are analyzed in detail as, for instance, the intriguing interference effect between the conventional and virtual echo contributions to the total HSPE signal. Furthermore, it is shown that from HSPE measurements the instantaneous frequency of the emitted (nonlinear polarization induced) signal can be derived and that its time-dependent shift projects directly onto the dynamical Stokes shift and the energy reorganization process of the coupled solute-solvent complex. It will also be shown that the experimental data can be simulated using the multimode Brownian oscillator model. (C) 1998 Elsevier Science B.V. All rights reserved.