Symmetry of nonlinear optical response to time inversion of shaped femtosecond pulses as a clock of ultrafast dynamics

Pulse shaping of femtosecond lasers causes frequency and intensity changes in the electromagnetic field; these changes can be symmetric or asymmetric with respect to time. Within the first few femtosecond, the nonlinear optical response is purely electronic. The nuclear response follows soon after that and continues for hundreds of femtoseconds. Differences in the nonlinear optical response caused by time inversion of laser pulses are explored here on a number of different systems. The change in behavior is first probed in a solution of IR144 for different laser intensities which shows a distinctive asymmetry upon time inversion in the fluorescence signal above the linear response regime. Two-photon induced fluorescence of a rhodamine B solution at high intensities is found to have a symmetric phase dependence which becomes asymmetric because of self phase modulation and self focusing at higher laser intensities. Surprisingly, the fragmentation and ionization of isolated para-nitrotoluene molecules was found not to depend on time inversion of the shaped laser pulses. However, from pulse shaping experiments we can glean the relative timing of the fragmentation processes. Results from this study show that the response of a system to time inversion of shaped femtosecond pluses provides an internal clock for ultrafast physicochemical processes occurring during laser-matter interactions.