Controlling Conversion From Ultracold Atoms To Molecules And Magnetic Feshbach Resonances By Laser

The core problem of the ultracold physics is the prepare of the ultracold molecule and its quantum control. Up to now, controlling the collision reaction by using the electromagnetic field is considered as the most effective technique for the production of ultracold molecule. In this thesis, we investigate theoretically the control of atom-molecule conversion and mag-netically induced Feshbach resonances by using the density theory and mean field theory. It is summarized as follows:(1) We demonstrated theoretically preparing ultracold Rb2 molecules by using the multiple-photon adiabatic passage and the coherent population trapping (CPT) state of the atom-molecule conversion reactive process is obtained theoretically. Through solving the Gross-Pitaevskii(GP) equation under the mean-filed approximation, we find that the multiple-photon adiabatic passage can be used to prepare ultracold molecules efficiently. The maximal prepared efficiency of 92.3% is obtained for the given optimized laser field. We find the the high-intensity laser is needed to overcome the adverse impact of the nonlinear effect and the decay effect. The suitable ratio between the first and second laser fields is given to achieve the high prepared efficiency. Moreover, We investigate the influence of the delay time between the pulses on photoassociation.(2) We theoretically investigate how to prepare and detect ultracold molecules via the multiple-photon adiabatic passage by solving the Liouville equation. The ultracold molecules formed in high vibrational states via Feshbach Resonances are transferred into the state v= 0 via three intermediate vibrational states. Numerical analysis by using the experimen-tal data for the Cs2 molecule shows that the multiple-photon adiabatic passage can realize the preparing and detecting of the ultracold molecules efficiently. Both the preparing and detecting efficiencies depend strongly on the Rabi frequencies of four laser pulses. Mean-while, we find that the decays of relevant states obviously reduce the preparing and detecting efficiencies.(3) We demonstrated theoretically that two-color field can be used to modulate mag-netically induced Feshbach resonance in Bose gas. Two-color field coupling the ground state and excited state can change the scattering length and reduce the two-body loss coefficient. The analytical expression of scattering length is derived by solving the Heisenberg equation.The scattering length can be modified by changing the Rabi frequencies or the frequency of the optical field.