Coherent Control Of Femtosecond CARS By Pulse Shaping

Coherent anti-Stokes Raman scattering (CARS) is a nonlinear four-wave mixing process, which has been successfully employed to study molecular constants, collisional parameters, local structure formation, molecular temperature, vibrational imaging and remote sensing because of high sensitivity against fluorescence background, high collection efficiency and high spatial resolution. However, femtosecond-CARS is faced with two inevitable drawbacks due to the high laser intensity and broad laser spectrum, one is the existence of a strong non-resonant background from electronic nonlinear contributions and the other is low selectivity between neighboring Raman levels when multiple Raman levels are simultaneously excited, which greatly reduce the detection sensitivity and spectral selectivity of the femtosecond-CARS. So, how to effectively eliminate the non-resonant background signal and improve the selective excitation between neighboring Raman levels is the crucial issue for femtosecond-CARS. In this dissertation, we use the coherent control strategy based on the femtosecond pulse shaping technique to realize the intensity enhancement, spectral narrowing, selective excitation and high spectral resolution of femtosecond-CARS.In the first section, we reviewed the quantum coherent control idea, introduced the basic principle of pulse shaping technology and coherent anti-Stokes Raman scattering, and analyzed the significance and progress of the coherent control in femtosecond-CARS.In the second section, we introduced the theory model of femtosecond-CARS. By a π phase step modulation and other functions in the probe pulse, we obtained the enhancement of the CARS signal in benzene solution and the improvement of the signal-to-noise ratio. By supplementally applying a cubic phase modulation in the pump pulse, we realized the further narrowing of the femtosecond-CARS spectrum and the improvement of the spectral resolution.In the third section, we realized the selective excitation among different Raman levels in methanol and dibromoethane solution by applying a π or double-π phase step modulation in pump pulse and a π phase step modulation in probe pulse.In the fourth section, we propose a feasible scheme to achieve a high-resolution two-pulse femtosecond-CARS spectrum by shaping both the pump and probe pulses with a rectangular amplitude modulation.