Selective spectral filtering of molecular modes of β-carotene in solution using optimal control in four-wave-mixing spectroscopy

Femtosecond laser pulses are spectrally broad and therefore coherently excite several molecular modes. While the temporal resolution is high, usually no mode-selective excitation is possible. In this work, we demonstrate that, by phase shaping one of the two pulses, which excite the vibrational modes of beta-carotene within a coherent anti-Stokes Raman scattering (CARS) process, specific modes can be enhanced or suppressed. Using the pulse shaper setup in a feedback-controlled closed-loop optimizes this mode selection. Here, the ratio of signal intensities observed in the CARS spectrum serves as a feedback function for an evolutionary algorithm responsible for the optimization. The optimized phase structure of the femtosecond pulse is characterized using the frequency resolved optical gating (FROG) technique to get an insight into the optimization process. Furthermore, it is shown that the temporal resolution after optimization is still high enough to investigate the ultrafast molecular dynamics. The suppression or relative enhancement of vibrational modes persists over the entire coherence time. Copyright (c) 2006 John Wiley & Sons, Ltd.