Summary form only given. Evaporative cooling of atoms in an optical trap has recently received renewed attention as a means for obtaining degeneracy in an optically trapped bosonic Cs vapor as well as a fermionic Li gas. We have developed a new model describing the evaporative cooling process for atoms confined in a time-dependent optical potential formed by a single focused Gaussian laser beam. We find that a substantial increase in the phase-space density can be obtained by adiabatically reducing the trap laser power and thereby the trap well depth as a function of time. Since we are specifically interested in the evaporative cooling of fermionic /sup 6/Li atoms, we have included the effect of Fermi statistics in the model. Although the collision rate is suppressed as the temperature T is reduced below the Fermi temperature T/sub F/, we find that values of T/T/sub F//spl Lt/1 can be achieved for suitable initial conditions.
We present an evaporative cooling technique for atoms trapped in an optical dipole trap that benefits from narrow optical transitions. For an appropriate choice of wavelength and polarization, a single laser beam leads to opposite light shifts in two internal states of the lowest-energy manifold. Radio-frequency coupling between these two states results in evaporative cooling at a constant trap stiffness. The evaporation protocol is well adapted to several atomic species, in particular to the case of Lanthanides such as Er, Dy, and fermionic Yb, but also to alkali-earth metals such as fermionic Sr. We derive the dimensionless expressions that allow us to estimate the evaporation efficiency. As a concrete example, we consider the case of
162Dy and present a numerical analysis of the evaporation in a dipole trap near the ${J}^{\ensuremath{'}}=J$ optical transition at
832nm. We show that this technique can lead to runaway evaporation in a minimalist experimental setup.
We derive scaling laws for the number of atoms, collision rate, and phase-space density as a function of trap depth for evaporative cooling in an adiabadically lowered optical trap. The results are in excellent agreement with a Boltzmann equation model and show that very large increases in phase-space density can be obtained without excessive slowing of the evaporation rate. Predictions are in reasonable agreement with a recent experiment that achieves Bose-Einstein condensation by evaporation in an optical trap. We also discuss evaporation of fermionic mixtures and explain why Pauli blocking does not strongly inhibit cooling.