Efficient intensity-gradient cooling of atoms in a weak standing-wave hollow-beam trap

We propose a trap scheme for cold alkali-metal atoms with an efficient intensity-gradient induced Sisyphus cooling in a weak standing-wave hollow-beam gravito-optical trap, which is composed of the interference of two well collimated, counterpropagating doughnut hollow beams with an intensity difference and a plug beam. We calculate the intensity distribution of the weak standing-wave hollow-beam field and its intensity gradient one, and find that such an optical dipole trap with an extremely high intensity gradient is desirable to realize an efficient intensity-gradient cooling for alkali-metal atoms in the trap. We also calculate the optical potentials, instantaneous dipole forces and spontaneous emission rates for a three-level dressed atom and study the dynamic process of intensity-gradient cooling of 87Rb atoms in the weak standing-wave hollow-beam trap by Monte Carlo simulations. Our study shows that the minimum optical potential at each node in our dipole trap is high enough to trap almost all cold atoms with a temperature of 120 μK released from a standard magneto-optical trap, and an ultracold 87Rb atomic sample with a temperature of ∼0.73 μK can be obtained in the trap. Starting from this stage, an all-optical Bose-Einstein condensation (BEC) could be realized by using the optical-potential evaporative cooling