Controlling Orientation Of Diatomic Molecule Utilizing Ultrafast Laser Pulse

The interaction between laser and matter is a very important research subject in the optical field. In this dissertation, field-free molecular orientation based on the rigid rotor model is studied by solving Liouville equation under ultrafast laser pulse. The main works are summarized as follows:(1) We proposed a scheme for achieving field-free orientation of CO molecule steered by the combination of a non-resonant two-color femtosecond laser pulse and time-delayed THz laser pulse. It is shown that the molecular orientation degrees can be significantly enhanced by applying this scheme compared with the single femtosecond or THz laser pulse. The effects of the intensity of two-color femtosecond laser pulse and the delay time between the femtosecond laser pulse and the THz laser pulse on molecular orientation are discussed. The molecular orientation also depends on the carrier-envelop phase of the THz laser pulse and the initial rotational temperature of the molecule. By optimizing the electric field intensity of two-color femtosecond laser pulse, the delay time between the femtosecond laser pulse and THz laser pulse and the carrier-envelop phase of the THz laser pulse, the molecular orientation degrees can be enhanced to some extent. Efficient molecular positive and negative orientation degrees can be obtained by utilizing the two-color femtosecond laser pulse with lower electric field intensity together with the THz laser pulse. The temperature has a large influence on the molecular orientation. With the rise of the temperature, the molecular orientation degrees decrease rapidly.(2) We proposed a method to simulate field-free orientation of CO molecule by utilizing a non-resonant two-color shaped laser pulse with a slow turn on and rapid off together with a time-delayed THz laser pulse train. The molecular positive and negative orientation degrees can be significantly enhanced by applying this method compared to the single two-color shaped laser pulse and THz laser pulse train. The effect of the pulse number of THz laser pulse on the molecular orientation is studied. The influence of the delay time between two-color shaped laser pulse and THz laser pulse train are also discussed. By optimizing pulse number and the delay time, the molecular orientation degrees can be improved to a large extent. It is shown that high efficiency molecular orientation degrees can be obtained by utilizing two-color shaped laser pulse with lower electric field intensity together with THz laser pulse train with small pulse number. The method of two-color shaped laser pulse together with THz laser pulse train is a feasible scheme which can be used to reduce the pulse number of THz laser pulse and enhance molecular orientation. Compared with the single THz laser pulse train, the combination of both laser pulses can be better applied in the real experiment. With the increase of the temperature, molecular orientation degrees decrease gradually, field-free positive and negative orientation degrees of CO molecule are greater than 0.5at T=10K.