Excessive Levitation For The Efficient Loading Of Large-volume Optical Dipole Traps

With the development of laser cooling technique,the microcosmic trapping and controlling of ultracold atoms has become a well-developed technique,and it represents a valuable tool for many fundamental and applied researches in atomic,molecular and photonic physics.When the evaporation cooling had been applied to the obtained ultracold atomic sample,the atomic sample with a lower temperature,namely Bose-Einstein condensate(BEC)and quantum degenerate Fermi gases could be achieved and has many important applications,such as precision measurement,artificial gauge field.To obtain the atomic sample with a sufficiently low temperature,it needs to load ultracold atoms into a magnetic,optical or magneto-optical trap and therefore we can control them accurately.In a large number of experiments,many people would like to load as many atoms as possible into a large volume crossed optical dipole trap.Considering the large mass of some atoms,the magnetic levitation is needed to the atomic compensate the gravity and to suppress the anti-trapping potential caused by gravity for a higher loading rate.At present,many research groups have used magnetic levitation to load ultracold atoms into optical dipole traps and then obtain Bose-Einstein condensation by evaporative cooling.Especially for cesium atoms with the large mass and special collision properties,it is necessary to use magnetic levitation to load the cooled atoms into optical dipole traps efficiently.However,there is an easily ignored process during the magnetically levitated loading process,where the magnetic force begins to increase but has not yet compensated for the gravity completely.As a result,the residual gravity always accelerates the atoms and thus heats the atomic sample.In this paper,we study the heating effect of ultracold cesium atoms in the non-equilibrium,magnetically levitated loading process.We present the theoretical variations of both acceleration and velocity with levitation time and magnetic field gradient,and measure the evolution of the number of trapped atoms with the excessive levitation time at different magnetic field gradients.The dependence of the number of atoms on the magnetic field gradient is also measured for different excessive levitation times.The main work contents of this paper are as follows:1.The magnetically levitated optical dipole trap is studied theoretically.Then the acceleration and velocity of ultracold atoms under different magnetic field gradient and excessive levitation times are calculated.The theoretical results are analyzed finally.2.In our experiments,cesium atoms are cooled and trapped by magneto-optical trap.The compressed magneto-optical trap,optical molasses and degenerated three-dimensional Raman sideband cooling are used for ultracold atomic sample.The atomic gravity is compensated by adjusting the magnetic field gradient,so that the ultracold atomic sample is loaded into the large volume crossed optical dipole trap using the magnetic levitation.3.We study on the influence of the excessive levitation magnetic field gradient and time on the loading rate,and effectively increase the loading rate of optical dipole trap through the excessive levitation.4.The relationship between the excessive levitation magnetic field gradient and the time corresponding to the maximum loading rate of optical dipole trap is studied.The experiment shows good agreement with theory.Among the work of this thesis,we have carried out the theoretical and experimental researches on the magnetic levitation optical dipole trap of ultracold cesium atoms.The heating effect during magnetic levitation loading process can be suppressed by excessive levitation.The loading rate of a large volume crossed optical dipole trap is further improved.