Düsseldorf 2007 – scientific programme
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MO: Fachverband Molekülphysik
MO 21: Femtosecond Spectroscopy II
MO 21.5: Talk
Tuesday, March 20, 2007, 11:45–12:00, 6D
Mode-Selective Excitation with Tailored Femtosecond Laser Pulses under Electronically Resonant and Nonresonant Excitation Conditions: The Role of Phase and Amplitude Modulation — •Jakow Konradi1, Atila Gaál2, Abraham Scaria1, Vinu Namboodiri1, and Arnulf Materny1 — 1International University Bremen (Jacobs University Bremen as of spring 2007), Germany — 2International Laser Center, Bratislava, Slovak Republic
Coherent anti-Stokes Raman scattering (CARS) spectroscopy with femtosecond laser pulses is a powerful tool for the investigation of vibrational dynamics on an ultrashort time scale. However, the gain of temporal resolution entails a poor spectral resolution due to the inherent spectral width of the femtosecond excitation pulses. Recently, we have demonstrated that the nonlinear four-wave mixing signal extremely sensitively depends on the “chirp” of the exciting and probing pulses. The application of a self-learning loop approach for the optimal shaping of the femtosecond laser pulses allows for a relative (and absolute) enhancement or suppression of selected vibrational modes. Here, the ratio of signal intensities of different Raman lines observed in the CARS spectrum served as feedback signal for an evolutionary algorithm finding the best setting for the pulse shaper. While in our first experiments solely a modulation of the phases of the spectral pulse components was applied, in our present contribution we will discuss results obtained from an optimization varying phase as well as amplitudes. The optimization of the CARS spectra was performed both under electronically resonant and nonresonant excitation conditions.