Application of feedback-controlled pulse shaping for control of CARS spectra: the role of phase and amplitude modulation

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). Copyright (C) 2007 John Wiley & Sons, Ltd.