Time Resolved Observation of Resonant and Non-resonant Contributions to the Nonlinear Susceptibility

In recent years, the optical third order nonlinear susceptibility X (3) has attracted considerable attention. Several three-wave mixing techniques have been developed to determine X(3) coefficients [ 1 -5 ] . Various contributions to the nonlinear susceptibility originating from single frequency resonances [6], Raman (difference frequency) resonances and two-photon resonances were inferred from measurements of the frequency dependence of X(3) and/or the polarisation properties [3,5] of the applied light fields. Nonlinear refractive indices far from resonance were measured by interferometric techniques and by self-phase modulation [7, 8]. Most experiments were made under quasi-stationary conditions using light pulses of a duration of several 10 _9 s. On the other hand, there exist extensive investigations of light-matter interactions performed under transient conditions [9 -12 ] . The properties of the coherent material excitation were measured as a function of time yielding new information on the dephasing time T 2 and the inhomogeneous broadening of the molecular transition. In these experiments, the resonant excitation of strong Raman lines in pure systems (liquids, gases) was studied; the resonant part of X (3) dominates and the other (nonresonant) contributions were negligible.