Abstract

New data are presented on the optical Kerr effect (OKE) of several organic molecular liquids using 65-fs optical pulses centered at 633 nm. Four dynamically distinct responses are observed in every liquid composed of molecules that possess a permanent polarization anisotropy. These include an instantaneous response that follows the laser pulse intensity autocorrelation, an ultrafast component with a 1/e time constant of < 170 fs, an intermediate response with τ_1/e of 400-600 fs, and a slower response commonly associated with orientational diffusion. In addition to these nonresonant contributions to nonlinear susceptibility, low-frequency intramolecular vibrations may be resonantly driven by the different Fourier components of the femtosecond laser pulse. In such cases the temporal evolution of these vibrational modes is directly observed superimposed on the nonresonant signal contribution. The OKE transients are discussed in terms of a simple model for molecular motion with inertial effects included explicitly. This analysis, in conjunction with the wealth of information available from frequency domain light-scattering experiments and molecular dynamics simulations, enables us to make definitive statements as to the origin and nature of the observed subpicosecond responses.

© 1987 Optical Society of America