Experiments to Control Atom Number and Phase-Space Density in ...

used for loading, is above 2 mK. The trap depth of the optical dipole trap, however, is
only about 700 µK. A simple model estimates the equilibrium atom number trapped in
an optical dipole trap [28]
N = N0 F
U0 U0
= nMOTVtrap F , (8.1)
kBT kBT
whereN0 is the number of atoms within the volumeVtrap of the dipole trap at the density
nMOT = N0/Vtrap. F[q] is given by
with
F[q] = q3/2
2
g1(x) = (−ln(1−x))3/2 (1−x) 1/2
x 2
1
0
16
π
dxx 2 g1(x)exp[q(1−x)], (8.2)
1
Figure 8.2 shows F as a function of U0/kBT.
F
15
10
5
0
duu 2 exp[(ln(1−x))(1−u 2 )]−1. (8.3)
0
0 1 2 3 4 5
U 0kBT
Figure 8.2: Dipole trap loading efficiency as a function of U0/kBT.
The number of atoms loaded into the into the CO2 dipole trap is greatly enhanced
as the temperature of atoms in the MOT is reduced. It is therefore necessary to compress
the MOT in order to reduce the temperature and increase the number of atoms in the
optical dipole trap, even though compressing the MOT leads to atom loss. Typically
the compression sequence is run after the MOT is loaded at a detuning of 38 MHz. The
Zeeman slower beams and field are turned off and the atomic beam shutter is closed.
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