Femtosecond dynamics of thin films by six-wave mixing

The ultrafast dynamics of 10-μm-thick solutions of 4-dimethylamino-4′ nitrostilbene dye are measured by time-resolved non-degenerate six-wave mixing. The six-wave mixing origin of the optically induced second harmonic signal is confirmed. The time domain results are analysed in terms of a higher-order grating scattering model. The slowest relaxation time is shown to arise from orientational relaxation, through its viscosity dependence. It is shown that the observed orientational dynamics reveal the first and third moments of the molecular distribution function. These moments are not available from other experiments. It is also found that orientational relaxation cannot account for all the relaxation kinetics observed in polar solvents. It is proposed that the early time dynamics reflect a time-dependent change in the solute hyperpolarisability in polar solvents.