Selective Excitation Of Femtosecond CARS Based On Adaptive Pulse Shaping Technology

The selective excitation based on adaptive pulse shaping is one of the hot and developing fields in the world recently. With the development of femtosecond laser techniques, Femtosecond CARS has been widely applied on chemical reaction control, biologic moleculars micro-imaging and local structure formation and temperature measure. Due to the broadband spectrum of femtosecond pulse, it can simultaneously excite many vibrational energy levels, it is difficult to eliminate the nonresonant background signal and selectively excite between the neighboring energy levels. The adaptive pulse shaping technique, profited from the feedback trait, can help people to find the proper pulse shape for a given goal and achieve the desirable outcomes by optimal search in a given space controlled by feedback control based on genetic algorithm. This technique provides an effectual method in experiment for selective excitation of fs-CARS.There are six parts in this thesis. In the first part, we simply introduce the basic conceptions of coherent anti-Stokes Raman spectroscopy (CARS), pulse shaping and coherent control, and clarify our work. In the second part, we show the basic theory of CARS. In the third part, we firstly introduce the significance and latest developments of CARS; then present the basic theories of adaptive pulse shaping and 4f pulse shaping setup; finally introduce two methods on ultra-fast laser measurement in our experiment. In the fourth part, we successfully achieve the selective excitation from benzene liquid by adaptive pulse shaping based on genetic algorithm, and then the physical mechanism for selective excitation of Raman vibrational modes is experimentally investigated by tailoring the pump pulse in the frequency domain. Finally we propose some feasible projects on the program according to the results of our experiments. In the fifth part, we achieve selective excitation in CARS with high spectral resolution by two different experiments. The first experiment, we realize the selective excitation between the v_s(CH₃) and V_AS(CH₃) in methanol liquid with the energy difference of 116 cm^-1. The second experiment, we realize the selective excitation in the mixture of C₆H₆ (992 cm^-1) and C₆D₆ (945 cm^-1), and their energy difference is only 47 cm^-1. Compared with the previous experiment in the mixture of CH4 (1388 cm^-1) and CHCl₃ (1889 cm^1-), the energy difference reduced about an order. The last part is a conclusion and expectation of this thesis.