Instantaneous and noninstantaneous nonlinear effects in femtosecond pulse propagation

Propagation of intense femtosecond pulses in solids and liquids is investigated experimentally and numerically. A brief review of experiments in fused silica that reveal complicated spatio-temporal dynamics including temporal pulse splitting is presented. Experiments in methanol show the expected effects of linear dispersion and an instantaneous Kerr nonlinearity as well as a contribution from a noninstantaneous nonlinearity with a response time on the order of 10 femtoseconds. Propagation in these two systems is modeled using a modified (3 + 1)-dimensional nonlinear Schroedinger equation (NLSE) that includes nonlinear shock and space-time coupling terms and a (1 + 1)-dimensional NLSE that includes a response time of the nonlinear part of the refractive index