Femtosecond Temporal Phase-Resolved Nonlinear Optical Spectroscopy in Molecules with Lock-in Enabled Phase Tracking

We describe an experiment to measure the emitted real-time electric field from an ultrafast third-order nonlinear optical interaction in molecules, using a phase-tracked spectral interferometry scheme. By combining a software lock-in amplification based spectrometer with spectral interferometry, we measure the electric field of the nonlinear optical signal from rotationally excited gas-phase molecules. The lock-in spectrometer allows selective measurement of signals of interest with improved signal-to-noise ratio, while rejecting any unwanted incoherent background. The nonlinear optical signal interferes with a known reference pulse on the spectrometer, which allows measurement of ultraweak signal electric fields. Further, we show that lock-in detection enables correction of slow interferometric drifts by utilizing a multidimensional measurement space. Thus, interferometric stability is achieved without the need for active stabilization, which typically utilizes an independent phase drift measurement. We present data from an experiment in impulsively aligned molecules that demonstrates the important features of our scheme. The scheme can be applied to study ultrafast dynamics in laser excited systems in the gas, liquid, and solid phases of matter.