Studies on cold atoms trapped in a Quasi-Electrostatic optical dipole trap

Abstract

We discuss the results of measurements of the temperature and density distribution of cold Rubidium atoms trapped and cooled in an optical dipole trap formed by focussed CO2 laser beams at a wavelength of 10.6 μm from a cold, collimated and intense atomic beam of flux 2 × 1010 atoms/s produced using an elongated 2D+MOT. A large number of rubidium atoms (⩾ 1010) were trapped in the MOT and the number density of atoms were further increased by making a temporal dark MOT to prevent density-limiting processes like photon rescattering by atoms at the trap centre. Subsequently, between 107 to 108 cold atoms at a temperature below 30 μK were transferred into a Quasi-Electrostatic trap (QUEST) formed by focussed CO2 laser beams at the MOT centre. Both single beam and crossed dual beam dipole traps were studied with a total output power of 50 W from the CO2 laser with focal spot sizes less than 100 microns. Various measurements were done on the cold atoms trapped in the dipole trap. The total atom number in the dipole trap and the spatial atom number density distribution in the trap was measured by absorption imaging technique. The temperature was determined from time-of-flight (TOF) data as well as from the absorption images after ballistic expansion of the atom cloud released from the dipole trap. The results from measurements are used to maximize the initial phase-space density prior to forced evaporative cooling to produce a Bose-Einstein Condensate.