Investigation of ultracold rubidium atoms in a pulsed far off resonance trap.

摘要

This dissertation reports on the design, construction, and investigation of a pulsed optical dipole force trap which uses laser light to confine ultracold rubidium (Rb) atoms. Because the laser frequency is detuned far from the atomic resonance frequency, the optical dipole force trap is also called a "far-off-resonance trap" (FORT). The use of pulsed laser light to create an optical trap may find application in expanding the number of atomic species which can be confined. The experiments reported here are principally aimed, however, at understanding the physics of pulsed FORT dynamics in anticipation of using the free electron laser (FEL) at Jefferson Lab (Jlab) to construct a FORT. The Jlab FEL will provide a tunable, high power laser light source enabling the realization of spatially large and/or energetically deep traps which are not presently accessible with table-top laser sources. Here, a mode-locked Nd:YAG laser is used as a pulsed laser source. Since the conservative optical dipole force trap does not cool Rb atoms, ultracold atoms must be loaded into the FORT. The optical dipole force trap is formed at the focus of a Gaussian, 1.06 mum Nd:YAG laser beam which is located at the center of a high vacuum chamber and superimposed onto the center of a pre-cooled Rb atom cloud that has been previously accumulated in a magneto-optic trap (MOT). The performance of the pulsed FORT is compared to a continuous wave mode (cw) FORT which is built using the same laser beam; the operation of the two kinds of FORTs can be switched easily without disturbing the experimental alignment. The dependencies of FORT loading efficiency to FORT and MOT parameters such as FORT laser power, loading time, storage/holding time, detuning of the primary MOT laser frequency, and repump laser intensity are investigated. There are about 1.5x10 7 ultracold rubidium atoms in the MOT. At 7 Watts FORT laser power, about 8% of the atoms are loaded successfully into the cw FORT and about 5% into the pulsed FORT under similar, but not identical, conditions. In most respects both the cw and pulsed FORTs show comparable behavior. The behavior of both FORTs depends strongly on FORT laser power. As expected, at higher power, more atoms can be loaded into the FORT. Both FORTs also depend on the loading time and holding time. (Abstract shortened by UMI.)