Geometrical effects in the loading of an optical atom trap

Loading of atoms contained in a magneto-optic trap into an optical trap formed with the focused laser beam from a $\mathrm{C}{\mathrm{O}}_2$ laser is examined. We show that while the loading efficiency is very sensitive to the volume of the optical trap, it is insensitive to the total laser power once a certain threshold is reached. Based on these observations a time-averaged optical atom trap is realized to compensate for the small capture volume of tight traps which are required for efficient evaporative cooling. Up to a 100% increase in the trap population is observed for the time-averaged traps. We also study the effect on temperature and trap population of changing a time-averaged optical trap back to a static trap. Our results provide a basis for increasing the number of atoms in Bose-Einstein condensates produced using all-optical techniques and show that a high-power laser is not a prerequisite for such an increase.

Figure 1

Experimental schematic. The optical arrangement used to create the $\mathrm{C}{\mathrm{O}}_2$ FORT beams is depicted in (a). In (b) the geometry of the FORT and MOT beams with respect to the vacuum chamber is illustrated. Additional MOT beams which propagate into and out of the plane of the paper are not shown.

Figure 2

Experimental data showing the number of atoms contained in the FORT vs power in the $\mathrm{C}{\mathrm{O}}_2$ beam. The power is focused to a $35\pm 5\mu \mathrm{m}$ waist.

Figure 3

Experimental data showing the number of atoms vs beam waist. The total power in $\mathrm{C}{\mathrm{O}}_2$ beam was $36\mathrm{W}$.

Figure 4

Experimental data representing the number of atoms trapped in the FORT vs the intensity of the repump light during the FORT loading. The beam waist was $35\pm 5\mu \mathrm{m}$ and power in the FORT beam was $36\mathrm{W}$. The initial repump intensity used to make the MOT was $0.16\mathrm{mW}∕{\mathrm{cm}}^2$. The square and open circle symbols represent the data for $\Delta \nu =0$ and $2\mathrm{MHz}$, respectively.

Figure 5

Experimental data showing the number of atoms vs sweeping period. The beam waist was $35\pm 5\mu \mathrm{m}$ and power in the FORT beam was $36\mathrm{W}$. The open circle, square, and star symbols represent the data for $\Delta \nu =1$, 2, and $3\mathrm{MHz}$, respectively.

Figure 6

Experimental data showing the number of atoms vs $\Delta \nu$. The beam waist was $20\pm 5\mu \mathrm{m}$ and power in the FORT beam was $20\mathrm{W}$. The square and circle symbols are the FORT population before and after damping, respectively.

Figure 7

Temperature of the atomic cloud vs frequency sweep. The open circle and square symbols are the cloud temperature before and after damping the sweeping amplitude.