Evaporative cooling in an optical dipole trap at 1 μm wavelength

Cold ensembles and Bose-Einstein condensates of alkali atoms in optical dipole potentials have been intensely studied in recent years. Most of these BEC experiments rely on producing the condensate by evaporative cooling in a magnetic trap with subsequent transfer to the optical potential. A smaller number of groups have also succeeded in achieving quantum degeneracy directly in a CO₂-laser trap. In optical traps, the evaporative cooling is done by ramping down the laser power of the trap lasers instead of using an RF-knife. In our approach, we perform evaporative cooling in a simple two-stage process with atoms collected and pre-cooled in a magneto-optical trap (MOT) and then directly transferred to a dipole trap whose intensity is ramped down in order to induce evaporation. We use a solid state laser to create a far-detuned trap for ⁸⁷Rb at a wavelength of 1030 nm instead of a CO₂-laser The dipole trap is realised by crossing two focused laser beams at an angle of 90°. This represents a simple way to create a nearly isotropic trap with tight confinement in all three dimensions.