Preparation And Investigation Of Ultracold Mixtures Of Lithium And Strontium

In this thesis,a machine for ultracold Li-Sr mixtures,designed and built by the au-thor as the main contributing student,is reported.Using this apparatus,gray molasses of⁶Li atoms,Bose-Einstein condensation of⁸⁴Sr atoms,and the first⁶Li-⁸⁸Sr mixture in optical dipole trap are created.In addition,we extract magnitude of the s-wave scattering length between⁶Li and⁸⁸Sr using interspecies thermalization measurement.The ultracold Li-Sr machine is composed of an ultra-high vacuum system,a laser system,a magnetic field system,and a control system.To make the vacuum system work well for both atomic species,a two-component oven,a common Zeeman slower and an octagonal glass science chamber with superb optical accesses,are designed by the author.An apparatus for ultracold atomic mixture like we design and constructed requires relatively more complex laser systems compared to a single-species one.The?near?resonant lasers for cooling,trapping,and detecting atoms and far-detuned lasers for providing optical dipole traps built for Li and Sr atoms are presented in detail.As for the magnetic field system,a compact and versatile magnetic coil setup is construct-ed and the magnetic field generated is stabilized by a current control feedback loop.In addition,we develop a method for fast switching-off of the coil current,which is essential for the success of gray molasses for⁶Li atoms.Using the apparatus above,we have realized magneto-optical trapping,gray mo-lasses,and optical dipole trap loading of⁶Li atoms,which result in the temperature reduction of atoms from 773 K in the oven to about 34?K.For strontium,the tempera-ture of the atomic gases is lowered to 1.8?K by adopting a two-stage magneto-optical trap?MOT?,i.e.a broad-linewidth MOT operating at 461 nm followed by a narrow-linewidth MOT operating at 689 nm.Subsequently,⁸⁴Sr atoms,of moderate scattering length and are suitable for efficient evaporative cooling,are Bose-Einstein condensed via rapid forced evaporation cooling in a crossed optical dipole trap.The atom number and temperature of typical condensates are 1.2×10⁵and 70 n K,respectively.Trans-fer efficiencies of nearly 100%are realized between the two stages of MOT,as well as between the narrow-linewidth MOT and dipole trap.Besides,Li and Sr atoms are transferred to one optical dipole trap,and the previously unknown interspecies s-wave scattering length between⁶Li and⁸⁸Sr is obtained to be|a6_Li-88_Sr|=(380_-100⁺¹⁶⁰)a₀.