Direct Electric Field Measurement of Femtosecond Time-Resolved Four-Wave Mixing Signals in Molecules

We measure the third order nonlinear response in molecules such as CO₂ and ethylene, by fully reconstructing the emitted signal electric field. Using a spectral interferometry-based technique for measuring ultraweak femtosecond pulses, called TADPOLE, we measure the real and imaginary parts of the nonlinear response in a single measurement. The nonlinear signal is generated using the Optical Kerr-effect (OKE), where a pair of femtosecond pulses (gate and probe) generate a signal that co-propagates with the probe beam having a polarization orthogonal to the probe polarization. Properties of the reconstructed signal electric field, such as the pulse envelope full-width half maximum and chirp of the temporal phase, can provide detailed information about ultrafast dynamics in these systems. We present results from measurement of nonlinear signal electric fields generated from purely electronic response in neutral CO₂ molecules and ionized ethylene molecules. A further extension of these experiments to measure higher order nonlinear response, such as the fourth order non-linear response in chiral molecules will be discussed.