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Journal of the Optical Society of America B

Improved trapping in a vapor-cell magneto-optical trap with multiple laser frequencies

Alastair G. Sinclair, Erling Riis, Michael J. Snadden

Published Dec 1, 1994• 13 citations
Publisher

Abstract

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.

DOI
10.1364/josab.11.002333

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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 present an experimental study of the number and density of trapped atoms in a vapor-cell Zeeman optical trap. We have investigated how the number (and therefore the capture rate) and density change with the trapping laser's beam diameter, intensity, and detuning and with the magnetic-field gradient of the trap. We have developed a quasi-one-dimensional numerical model that accurately predicts the number of trapped atoms for all conditions. We also have investigated chirping the laser frequency and trapping with broadband light, neither of which increase the number of trapped atoms.
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).
We have demonstrated an order of magnitude increase in the number and in the lifetime of magneto-optically trapped 87Rb^{87}\mathrm{Rb}87Rb atoms collected from a low-pressure vapor. The improvement results from the reduction of light-assisted collision losses and is achieved by shelving the atomic population into the lower hyperfine ground state. Large fractional populations of the lower hyperfine ground state have been obtained by blocking the hyperfine repumping light in the region of the trapped atoms and by application of laser light that optically pumps the atoms into that state. We have observed trap lifetimes in excess of 700 s at our highest vacuum levels. At the rubidium pressure that maximizes the product of the number and lifetime of trapped atoms we observe a lifetime of 240 s with 5\ifmmode\times\else\texttimes\fi{}107{10}^{7}107 trapped atoms, under conditions for which an ordinary magneto-optical trap has a lifetime of only 14 s with 3\ifmmode\times\else\texttimes\fi{}106{10}^{6}106 atoms.
We demonstrate a novel dual color magneto–optical trap (MOT), which uses two sets of overlapping laser beams to cool and trap 87 Rb atoms. The volume of cold cloud in the dual color MOT is strongly dependent on the frequency difference of the laser beams and can be significantly larger than that in the normal MOT with single frequency MOT beams. Our experiment shows that the dual color MOT has the same loading rate as the normal MOT, but much longer loading time, leading to threefold increase in the number of trapped atoms. This indicates that the larger number is caused by reduced light induced loss. The dual color MOT is very useful in experiments where both high vacuum level and large atom number are required, such as single chamber quantum memory and Bose–Einstein condensation (BEC) experiments. Compared to the popular dark spontaneous-force optical trap (dark SPOT) technique, our approach is technically simpler and more suitable to low power laser systems.
We demonstrate a novel dual color magneto—optical trap (MOT), which uses two sets of overlapping laser beams to cool and trap 87 Rb atoms. The volume of cold cloud in the dual color MOT is strongly dependent on the frequency difference of the laser beams and can be significantly larger than that in the normal MOT with single frequency MOT beams. Our experiment shows that the dual color MOT has the same loading rate as the normal MOT, but much longer loading time, leading to threefold increase in the number of trapped atoms. This indicates that the larger number is caused by reduced light induced loss. The dual color MOT is very useful in experiments where both high vacuum level and large atom number are required, such as single chamber quantum memory and Bose—Einstein condensation (BEC) experiments. Compared to the popular dark spontaneous-force optical trap (dark SPOT) technique, our approach is technically simpler and more suitable to low power laser systems.