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 bcarotene 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.

The feasibility of mode-selective excitation with broadband femtosecond laser pulses is demonstrated for toluene in liquid phase. A learning-loop optimal control scheme was applied to a stimulated Raman excitation process. Modifications of the phase shape of one of the exciting pulses resulted in dramatic changes of the mode distribution reflected in coherent anti-Stokes Raman spectra. An evolutionary algorithm guided the coherent excitation process to a selective enhancement or suppression of one or more vibrational modes over the complete coherence lifetime spanning several picoseconds. New ways of spectral filtering as well as exciting possibilities of mode-selective studying of chemical reaction dynamics are indicated.

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Femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) results in spectra that, as a function of the probe delay time, yield information about the dynamics of the coherently excited vibrational modes. A change of the shape of the exciting laser pulses has a dramatic influence on the spectral response. A feedback-controlled optimization of specific modes making use of phase and amplitude modulation of the Stokes laser pulse is applied to selectively influence the anti-Stokes signal spectrum. The role of phase and amplitude changes of the frequency components of the ultrashort pulse is analyzed. It can be demonstrated that the optimization process is clearly dominated by the effect of timing of the dispersed pulse segments. The modulation of the spectral amplitudes has only a small influence on the mode ratios. We conclude that mode focusing in time domain CARS spectroscopy can be achieved only by correctly setting the phases of the spectral pulse components (here demonstrated for the Stokes laser).

A feedback-controlled optimization in a femtosecond coherent-anti Stokes Raman scattering (CARS) process is applied to selectively excite or suppress vibrational modes in the gas and liquid phase. The optimal control experiments are performed on carbon disulfide and toluene molecules. Here our aim is to understand whether the interaction of the molecules with the surrounding medium affects the optimization process. The CARS excitation was chosen to be not in resonance with an electronic transition in the molecule but to excite different vibrational modes coherently. A pure phase modulation of the Stokes pulse resulted in changes of the ratio of the Raman lines observed in the nonlinear scattering spectrum. This could also be achieved when no temporal shift between the pump and Stokes laser resulted in a simple change of the Raman resonances. The relative intensities of the Raman lines could be changed more effectively in the liquid phase than in the gas phase. The higher density in the condensed matter, which hinders free rotation and makes interactions between molecules an important factor, obviously seems to influence the control mechanism.