All-optical runaway evaporation to Bose-Einstein condensation

We demonstrate runaway evaporative cooling directly with a tightly confining optical-dipole trap and achieve fast production of condensates of $1.5\times {10}^5$ ${}^{87}\text{R}\text{b}$ atoms. Our scheme uses a misaligned crossed-beam far off-resonance optical-dipole trap (MACRO-FORT). It is characterized by independent control of the trap confinement and depth allowing forced all-optical evaporation in the runaway regime. Although our configuration is particularly well suited to the case of ${}^{87}\text{R}\text{b}$ atoms in a 1565 nm optical trap, where an efficient initial loading is possible, our scheme is general and will allow all-optical evaporative cooling at constant stiffness for every optically trappable atomic or even molecular species.

Figure 1

(Color online) (a) Scheme of our MACRO-FORT. The confining beam crosses the wide beam at a distance of about $80\mu \text{m}$ from its center. This configuration decouples the control of the trap depth $U_{\text{trap}}$ from the control of the trap confinement. (b) Vertical potential energy cuts. The two curves correspond to the same power of 0.15 W in the confining beam whereas the wide beam powers are 8 W (light/blue curve 1) and 16 W (dark/red curve 2). It shows that the trap depth is reduced by increasing the wide beam power. The atoms, pulled upward from the confining beam waist, are lost in the direction of the wide beam, perpendicular to the figure, along which there is no confinement.

Figure 2

(Color online) Loading scheme into the wide trapping beam. (a) Light shifted levels of the $5S_{1/2}$ and $5P_{3/2}$ states of ${}^{87}\text{R}\text{b}$ under the influence of the wide beam at a power of 28 W. At 1565 nm, the $5P_{3/2}$ excited state of the MOT cooling transition is 42.6 times more shifted than the $5S_{1/2}$ fundamental state [17]. For clarity the hyperfine structure is not shown. During the very far-detuned molasses phase, the cooling laser is 200 MHz detuned and therefore remains to the red of the cycling transition even in the presence of the trapping laser. The repumping light on the contrary is on resonance in free space and is brought out of resonance by the dipole laser light. (b) Repumping rate as a function of position. The repumping is not efficient at the position of the dipole trap leading to a depumping effect leaving the atom into the “dark” $F=1$ hyperfine states.

Figure 3

(a) Optical power in the two trapping beams as a function of time during the evaporation. The solid (dashed) curve corresponds to the tightly confining (wide) beam. In the second part of the evaporation the tightly confining beam power is roughly constant whereas the power of the wide beam is increased to force the evaporation. [(b) and (c)] Phase-space density and collision rate as a function of time.