Theoretical Study On Optical Control Of Molecular Orientation And Magnetic Feshbach Resonances In Bose Gases

Molecular orientation and alignment have attracted the attention of many researchers because of their extensive applications in chemical reaction dynamics, high-harmonic gener-ation, strong-field ionization and photoelectron angular distribution. Moreover, for investi-gating the fundamental behavior of quantum matter, using external electromagnetic field to control the interaction between ultracold atoms has become a essential method. In this thesis, orientation of diatomic molecule with ultrafast laser field and optical control of magnetically induced Feshbach resonances in Bose gases are theoretically investigated. The main works contained in this thesis are as follows:(1) We present a theoretical scheme for achieving the field-free molecular orientation in dissipative media by a combination of femtosecond and THz laser pulses. Numerical calculations are performed by solving the quantum Liouville equation based on multilevel Bloch model. The molecular orientation degree is sensitive to the carrier-envelope phase of the THz pulse and the delay time between the two pulses. The orientation and the rotational population of CO molecules in dissipative environment are computed at different pressures and temperatures. The influence of pure decoherence on the molecular orientation is also discussed.(2) We theoretically investigate field-free molecular orientation by using an off-resonant two-color shaped laser pulse with a slow turn-on and rapid turn-off and a time-delayed THz laser pulse. It is shown that the molecular orientation can be greatly enhanced by applying this scheme compared to that by the two-color shaped or THz laser pulse alone. We discuss the effects of the rising time and the electric field amplitude of two-color shaped laser pulse on molecular orientation, and demonstrate that effective molecular orientation can be achieved by a moderate intensity laser pulse with a short rising time. Furthermore, by changing the delay time between the two-color shaped laser pulse and the THz laser pulse and the carrier envelope phase of the THz laser pulse, the molecular orientation can be enhanced or lowered in some degree.(3) We theoretically investigate optical modulation of magnetically induced Feshbach resonance in Bose gases with two optical fields. The two optical fields couple two ground states through an excited state. Compared with the usual single optical scheme, two optical fields can greatly suppress the inelastic loss resulting from spontaneous emission by the destructive quantum interference. Using the mean field theory, the analytical formula of the scattering length is obtained. The results show that the scattering length can be modified in a large range by changing the Rabi frequency or the optical field frequency. Meanwhile, the atom-molecule interactions also have effects on the scattering length in varying degree.