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Journal of Physics Conference Series

Optical control of a magneto-optical trap for cesium atoms

Swarupananda Pradhan, B. N. Jagatap

Published Sep 1, 2007• 0 citations
Publisher

Abstract

Loading of a magneto-optical trap for cesium atoms is studied in presence of an auxiliary (control) laser beam that is tuned to 6s 1/2 F=4 → 6p 3/2 F' = 3,4,5 hyperfine manifold. It is shown that the steady state number of trapped atoms is enhanced or suppressed depending on the frequency, intensity and position of the control beam in the trap. The overall result is to provide an ability to optically control the number of trapped atoms without having to change the parameters of the trap. Detailed parametric studies of the control laser assisted optical manipulation of a MOT are presented.

DOI
10.1088/1742-6596/80/1/012041

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•Sep 1, 2011•0 citations
Loading of a magneto-optical trap for cesium atoms is studied in presence of an auxiliary (control) laser beam that is tuned to 6s 1/2 F=4 → 6p 3/2 F' = 3,4,5 hyperfine manifold. It is shown that the steady state number of trapped atoms is enhanced or suppressed depending on the frequency, intensity and position of the control beam in the trap. The overall result is to provide an ability to optically control the number of trapped atoms without having to change the parameters of the trap. Detailed parametric studies of the control laser assisted optical manipulation of a MOT are presented.
We demonstrate enhancement in the number of trapped cesium atoms in a magneto-optical trap (MOT) using a control laser that illuminates only a small faction of the capture region of the trap without interacting with the cold cloud of atoms. The enhancement is maximized when the laser is slightly blue detuned with respect to the cooling transition. Trap loading curves point to approximately a twofold increase in the capture rate, which as a consequence results in the increase in the steady state number of trapped atoms. Enhanced loading is confirmed by MOT loading and decay curves taken under the modulation of the control laser beam. Optical pumping of the inaccessible Zeeman states into the stretched states is suggested as a possible mechanism.
With the help of light-emitting diodes at the center wavelength of 470 nm, we demonstrate that light-induced atom desorption (LIAD) can be used for flexibly controlling the loading of magneto-optical traps (MOT) of cesium atoms. Under an ultralow background pressure in a quartz cell without any wall coating, we show that low intensity blue light can be used to control the loading rates (from 200 to 4000 atoms/s) and the number of cesium atoms (on the order of 104{10}^{4}104 in our experiment) in a MOT without the use of dispensers or any secondary atom source. A theoretical model based on an atom loading rate equation is built which can simulate the magneto-optical trap loading process with LIAD. The theoretical results are in agreement with experimental data. Some important parameters of the vacuum system are determined accordingly. The decay time of the vacuum pressure is about 70 ms, which is much shorter than the usual vacuum system for experiments on atomic ensembles. It is very difficult to measure such short times by the traditional fluorescence detection of cold atoms due to the slow loading process. The change of desorption rate (on the order of 1013 atoms/s{10}^{13}\text{ }\text{atoms}/\text{s}1013 atoms/s) for the different desorption intensities is also determined based on the background pressure caused by noncesium atoms in the cell. According to the experimental data and theoretical calculations, we obtain a partial pressure of about $1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\text{ }\text{Torr}$ for the untrapped cesium atoms and about $6.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\text{ }\text{Torr}$ for the noncesium atoms at the moment the desorption light is turned off. The system is almost an ideal vacuum system for transferring atoms into another region for further experiments.
A small known number of cesium atoms is loaded from a magneto-optical trap into a far detuned, standing wave optical dipole trap. This trap is made of two counter-propagating Nd:YAG laser beams (/spl lambda/= 1064 nm) which give rise to a chain of potential wells with a depth of about 1 mK and an axial spacing of 532 nm. By fitting the axial intensity distribution with a Gaussian function, the positions of the individual atoms are determined automatically with a precision of 100 nm rms, mainly limited by photon statistics.
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.
The results of characterization of cold cloud of cesium (Cs) atoms trapped in a magneto-optical trap (MOT) have been presented. The MOT is a part of the Physics package of the Cs Fountain Clock being developed at National Physical Laboratory (NPL), India. The number of atoms, size and temperature of the cloud have been measured and calculated. It is also been investigated how the number of trapped atoms changes with the trapping laser’s beam intensity, detuning and magnetic-field gradient of the trap.