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Optics Letters

Atom number in magneto-optic traps with millimeter scale laser beams

Gregory W. Hoth, Elizabeth A. Donley, John E. Kitching

Published Feb 21, 2013• 28 citations
Publisher

Abstract

We measure the number of atoms N trapped in a conventional vapor-cell magneto-optic trap (MOT) using beams that have a diameter d in the range 1-5 mm. We show that the N is proportional to d(3.6) scaling law observed for larger MOTs is a robust approximation for optimized MOTs with beam diameters as small as 3 mm. For smaller beams, the description of the scaling depends on how d is defined. The most consistent picture of the scaling is obtained when d is defined as the diameter where the intensity profile of the trapping beams decreases to the saturation intensity. Using this definition, N scales as d(6) for d<2.3 mm but, at larger d, N still scales as d(3.6).

DOI
10.1364/ol.38.000661

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We measure the number of atoms N trapped in a conventional vapor-cell magneto-optic trap (MOT) using beams that have a diameter d in the range 1-5 mm. We show that the N is proportional to d(3.6) scaling law observed for larger MOTs is a robust approximation for optimized MOTs with beam diameters as small as 3 mm. For smaller beams, the description of the scaling depends on how d is defined. The most consistent picture of the scaling is obtained when d is defined as the diameter where the intensity profile of the trapping beams decreases to the saturation intensity. Using this definition, N scales as d(6) for d<2.3 mm but, at larger d, N still scales as d(3.6).
A novel method has been demonstrated for increasing the number of 85Rb trapped atoms in a vapor-cell magneto-optical trap. By using a second set of trapping laser beams overlapped with the original beams and detuned further from resonance, we obtained a doubling of the trap-filling rate as higher initial velocities were slowed. Blocking out the centers of these new beams and reducing the intensity in the original beams also allowed us to obtain a reduction in the light-induced loss rate. Overall we have obtained a fivefold increase in the number of trapped atoms. With 20-mm-diameter beams, ~1.4 × 109 atoms could be trapped. A Gaussian density profile with a peak density of ~2.2 × 1011 cm−3 was observed.
We investigate the scaling behavior of a very large magneto-optical trap (VLMOT) containing up to $1.4\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{4pt}{0ex}}{\mathrm{Rb}}^{87}$ atoms, loaded from a background vapor. By varying the diameter of the trapping beams, we are able to change the number of trapped atoms by more than 5 orders of magnitude. We then study the scaling laws of the loading and size of the VLMOT, and analyze the shape of the density profile in this regime where the Coulomb-like, light-mediated repulsive interaction between atoms is expected to play an important role.
We present a magneto-optical trap (MOT) design based on millimeter ball lenses, contained within a metal cube of 19-mm side length. The ball lenses create highly divergent conical MOT trapping beams. In our experiment and in trajectory simulations, we study the effect of beam divergence and light-intensity gradients on MOT performance. We trap approximately $4.2\ifmmode\times\else\texttimes\fi{}{10}^{5}{\phantom{\rule{0.2em}{0ex}}}^{85}\mathrm{Rb}$ atoms in our ball-lens MOT at a density of $3.2\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and a loading time of 1.3 s. Experimental measurements and the trajectory model are used to calibrate the trapping efficiency in the highly divergent light fields of the ball lenses. The advantages of the design include small lens and chamber sizes favorable to miniaturization, the absence of large-diameter optical-beam pathways, and a low laser-power requirement. These features are conducive to atom-trap geometries with well-defined electromagnetic boundary conditions, as required for ion and plasma traps, and for efficient shielding of black-body and dc stray fields.
As part of an effort to measure parity non-conserving atomic transition rates in a series of Cs radioisotopes, the authors are working on the efficient coupling of a magneto-optical trap (MOT) to a mass separator. They have established a test stand for optical trapping which is used for the off-line development of a high-efficiency MOT. Their trap features a dry-film coated cell with large (5 cm) windows to permit the use of large diameter, high power laser beams to increase the efficiency of the trapping process. Studies of the effect of detuning, magnetic field gradient, laser beam intensity and different beam sizes on the number of trapped atoms are presented.