Optimal geometry for efficient loading of an optical dipole trap

One important factor which determines efficiency of loading cold atoms into an optical dipole trap from a magneto-optical trap is the distance between the trap centers. By studying this efficiency for various optical trap depths $(2–110\mathrm{mK})$ we find that for optimum dipole trap loading, longitudinal displacements up to $15\mathrm{mm}$ are necessary. An explanation for this observation is presented and compared with other work and a simple analytical formula is derived for the optimum distance between the trap centers.

Figure 1

The time sequence of loading the ODT from the MOT.

Figure 2

(Color online) Loading of the $33\text{−}\mathrm{mK}$-deep dipole trap for different positions of the ODP laser focus with respect to the MOT center. (a) Fluorescence images of atoms in the ODT, vertically stretched for better visualization of the ODT potential. The elliptic contours mark the MOT positions ($0.66\mathrm{mm}$ radius) before switching on the ODT. The elongated contours represent the ODT equipotential surfaces $U=-2.5k_B{T}_{\mathrm{MOT}}$. (b) Total fluorescence from the ODT, proportional to the number of atoms loaded into the ODT, vs the focus position. Solid line: $P_{\mathrm{ODT}}=200\mathrm{mW}$, $\Delta \lambda =0.43\mathrm{nm}$; dashed line: $P_{\mathrm{ODT}}=400\mathrm{mW}$, $\Delta \lambda =0.86\mathrm{nm}$.

Figure 3

Optimum displacement of the optical dipole trap relative to the magneto-optical trap for maximum transfer of atoms versus the ODT depth. The black dots represent our experimental data. The solid line is calculated from Eq. (5) with the scaling parameter $\alpha =2.5$, as described in the text. The dashed line is the prediction based on the model of O’Hara et al. with no free parameters [12]. See also Sec. 4.

Figure 4

The cross sections of the equipotential surfaces of the ODT formed by a focused Gaussian beam. $z∕z_R$ is the distance along the beam in units of the Rayleigh length. $r∕w_0$ is the radial coordinate in units of the beam waist. $U_0$ is the trap depth. Dashed lines mark the positions of $\pm z_{\max }$ for $U=0.1U_0$.

Figure 5

The initial number of trapped atoms $N$ as a function of the MOT-ODT displacement $z$ predicted by the model of O’Hara et al. [12] for our experiment (trap $\text{depth}=33\mathrm{mK}$).

Figure 6

The geometrical superposition of the trap potentials and the influence of the ODT beam on the MOT. (a) The geometry of the MOT (represented by the $1∕e^2$ density outline) and the $33\text{−}\mathrm{mK}$-deep ODT (represented by the equipotential surface $U=-2.5k_B{T}_{\mathrm{MOT}}$—see text). The displacement of the traps is optimized for maximum atom transfer. (b) The detuning of the MOT repumper caused by the presence of the ODT beam, plotted along the dashed line in (a). The gray outline represents the atomic density distribution of the MOT. (c) Same as (b), but along the ODT beam axis $(r=0)$. Note the vertical scale has been expanded for better visualization.