Role of coherent coupling in the femtosecond time-resolved optical Kerr effect in simple liquids

Abstract

Time-resolved pump-probe spectroscopies in the picosecond and femtosecond time domains exhibit an apparently instantaneous signal when the pump and probe laser pulses are temporally and spatially overlapped. This signal is related to the coherence properties of the laser pulses as well as to the material response. Meaningful information concerning the microscopic material response can only be extracted following a quantitative assessment of the coherence contribution to the measured signal. We present new results on this instantaneous signal via the femtosecond time-resolved optical Kerr effect in a wide variety of simple room-temperature liquids including CS2, benzene and its monohalogenated analogs, and several polyhalogenated methanes. A dramatic effect of the laser pulse width on the amplitude of the instantaneous signal (which is well described as a scalar multiple of the laser pulse intensity autocorrelation) is observed. Purely coherent contributions to this instantaneous signal are related to the electric field cross-correlation of the pump and probe pulses and have a definite symmetry in the time delay between these pulses that depends on the phase of the local oscillator in our optically heterodyned detection scheme.