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arXiv:1009.2544

Loading characteristics of a microscopic optical dipole trap

P. Kulatunga, Tyler Blum, D. Olek

Published Sep 14, 2010• 0 citations
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Abstract

We report on an investigation of loading characteristics of deep microscopic dipole traps. The dipole trap is loaded from a low density magneto optical trap (MOT) containing $\approx 5\times 10^{6}$ atoms. We determine the loading parameters that maximize the trapped atom number for a trap of waist 5 μm with trap depths ranging from 3.5 mK to 10 mK. We determine the optimal trap loading conditions and the loading rates, loss coefficients and temperature of the trapped atoms under these conditions. We show that it is possible to load a few hundred to thousand atoms in dipole traps of depth 3.5 mK to 8.5 mK under the optimal loading conditions.

DOI
10.48550/arxiv.1009.2544

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We present a detailed experimental study of the physics involved in transferring atoms from a magneto-optical trap (MOT) to an optical dipole trap. The loading is a dynamical process determined by a loading rate and a density dependent loss rate. The loading rate depends on cooling and the flux of atoms into the trapping volume, and the loss rate is due to excited state collisions induced by the MOT light fields. From this study we found ways to optimize the loading of the optical dipole trap. Key ingredients for maximum loading are found to be a reduction of the hyperfine repump intensity, increased detuning of the MOT light, and a displacement of the optical dipole trap center with respect to the MOT. A factor of 2 increase in the number of loaded atoms is demonstrated by using a hyperfine repump beam with a shadow in it. In this way we load $8\ifmmode\times\else\texttimes\fi{}{10}^{6}{}^{85}\mathrm{Rb}$ atoms into a 1 mK deep optical dipole trap with a waist of $58 \ensuremath{\mu}\mathrm{m},$ which is 40% of the atoms initially trapped in the MOT.
We investigate the properties of the focused-beam magneto-optical trap (FBT) of Rb 85 atoms, which provides more optical accessibility in contrast to typical magneto-optical trap (MOT). We have characterised the relations between temperature, number and density of trapped atoms versus trap parameters. In this paper, we discuss some distinctive features of the FBT comparing to those of standard MOTs, and determine a suitable condition in which loading atoms into a far-off-resonance optical dipole trap (FORT) is possible. Although the trap can capture fewer atoms comparing to the standard MOTs, due to its asymmetric geometry, it may offer an alternative trap configuration for preparing single atom.
We analyze the operating regimes of a very small optical dipole trap, loaded from a magneto-optical trap, as a function of the atom loading rate, i.e., the number of atoms per second entering the dipole trap. We show that, when the dipole trap volume is small enough, a "collisional blockade" mechanism locks the average number of trapped atoms on the value 0.5 over a large range of loading rates. We also discuss the "weak loading" and "strong loading" regimes outside the blockade range, and we demonstrate experimentally the existence of these three regimes.
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We present a simple Fokker-Planck model for the evolution of the number of atoms and the spatial distribution in a CO2{\mathrm{CO}}_{2}CO2​ laser trap which is loaded from a magneto-optical trap (MOT). Deep CO2{\mathrm{CO}}_{2}CO2​ laser traps can achieve very high densities, but reach equilibrium slowly compared to shallow traps. Equilibrium times range from milliseconds for shallow traps to seconds in traps which are deep compared to the thermal energy. A universal function for the maximum number of trapped atoms is given, assuming that the optical dipole trap reaches thermal equilibrium with the MOT.
We present our technique to create a magneto-optical trap (MOT) for dysprosium atoms using the narrow-line cooling transition at 626 nm to achieve suitable conditions for direct loading into an optical dipole trap. The MOT is loaded from an atomic beam via a Zeeman slower using the strongest atomic transition at 421 nm. With this combination of two cooling transitions we can trap up to 2.0·10(8) atoms at temperatures down to 6 μK. This cooling approach is simpler than present work with ultracold dysprosium and provides similar starting conditions for a transfer to an optical dipole trap.