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Journal of Physics B Atomic Molecular and Optical Physics

Anisotropic magneto-optical trapping of atoms: capture efficiency and induced drift velocities

G. Dudle, N Sagna, P. Berthoud, P. Thomann

Published Oct 28, 1996• 9 citations
Publisher

Abstract

An anisotropic magneto-optical trap is studied as the potential source of a continuous beam of laser-cooled atoms. Capture efficiency and cloud temperature are measured experimentally and found to be lower than in an isotropic situation with the same cooling parameters. Intensity imbalance and static magnetic field are investigated as possible extraction mechanisms. While only small drift velocities are induced by means of intensity imbalance, velocities of tens of are observed with a static magnetic field; the shape of the observed two-peaked velocity distributions is related to the average depth of local light-shift-induced potential wells.

DOI
10.1088/0953-4075/29/20/019

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Journal of the Optical Society of America B•Jun 1, 1999•12 citations
An anisotropic magneto-optical trap is studied as the potential source of a continuous beam of laser-cooled atoms. Capture efficiency and cloud temperature are measured experimentally and found to be lower than in an isotropic situation with the same cooling parameters. Intensity imbalance and static magnetic field are investigated as possible extraction mechanisms. While only small drift velocities are induced by means of intensity imbalance, velocities of tens of are observed with a static magnetic field; the shape of the observed two-peaked velocity distributions is related to the average depth of local light-shift-induced potential wells.
Atomic clocks based on a continuous beam of cold atoms are a promising alternative to fountains mainly because the low atomic density of a beam would reduce collisional shifts to a negligible level; in addition dead times can be suppressed or greatly reduced. Several mechanisms allowing to continuously extract cold atoms from an anisotropic trap will be discussed, in particular atomic drift induced by a constant magnetic field and laser beam imbalance. Experimental results of trap loading times and time-of-flight measurements will be presented to assess the potential of these methods for the production of a continuous beam of cold atoms.
We report on a simple anisotropic magneto-optical trap for neutral atoms that produces a large sample of cold atoms confined in a cylindrically-shaped volume with a high aspect ratio (100:1). Due to the large number of trapped atoms, the laser beams that propagate along the optically thick axis of the trap to cool the atoms are substantially attenuated. We demonstrate that the resulting intensity imbalance produces a net force that spatially localizes the atoms. This limits both the trap length and the total number of trapped atoms. Rotating the cooling beams by a small angle relative to the trap axis avoids the problem of attenuation, and atoms can be trapped throughout the entire available trapping volume. Numerical and experimental results are reported that demonstrate the effects of absorption in an anisotropic trap, and a steady-state, line-center optical path length of 55 is measured for a probe beam propagating along the length of the trap.
This paper presents an experimental demonstration of light-induced evaporative cooling in a magneto-optical trap. An additional laser is used to interact with atoms at the edge of the atomic cloud in the trap. These atoms get an additional force and evaporated away from the trap by both the magnetic field and laser fields. The remaining atoms have lower kinetic energy and thus are cooled. It reports the measurements on the temperature and atomic number after the evaporative cooling with different parameters including the distance between the laser and the centre of the atomic cloud, the detuning, the intensity. The results show that the light-induced evaporative cooling is a way to generate an ultra-cold atom source.
 A detailed experimental and theoretical investigation of a magneto-optical trap for caesium atoms is presented. Particular emphasis has been placed on achieving high spatial number densities and low temperatures. Optimizing both of these together enables efficient evaporative cooling from a conservative trap, a procedure which has recently led to the first observations of Bose-Einstein condensation in a dilute atomic vapour. The behaviour of a magneto-optical trap is nominally determined by four independent parameters: the detuning and intensity of the light field, the magnetic field gradient and the number of trapped atoms. A model is presented which incorporates previous treatments into a single description of the trap that encompasses a wide range of its behaviour. This model was tested quantitatively by measuring the temperature of the cloud and its spatial distribution as a function of the four parameters. The maximum density was found to be limited both by the reabsorption of photons scattered within the cloud and by a reduction of the confining force at small light shifts. The nonlinear variation with position of the restoring force was found to be significant in limiting the number of atoms confined to a high density. A maximum density in phase space (defined as the number of atoms in a box with sides of dimension one thermal de Broglie wavelength) of (1.5 ± 0.5) x 10-5 was observed, with a spatial density of 1.5 x 1011 atoms per cm3. Cold collision losses from a caesium magneto-optical trap have been studied with the purpose of assessing their influence on spatial densities. In contrast to previous measurements of similar quantities, these measurements did not require the use of an ultra-low ( A study is presented of a modified magneto-optical trap in which a fraction of the population is shelved into a hyperfine level that does not interact with the trapping light. In this so-called dark magneto-optical trap, improved densities of nearly 1012cm-3 have been previously reported for sodium. The application of the technique to caesium is not straightforward due to the larger excited state hyperfine splittings. A simple theory for caesium is presented and its main predictions verified by measurements of density, number and temperature. A density of nearly 1012cm,-3 was indeed obtained but at a temperature substantially higher than in the conventional magneto-optical trap.
A detailed experimental and theoretical investigation of a magneto-optical trap for caesium atoms is presented. Particular emphasis has been placed on achieving high spatial number densities and low temperatures. Optimizing both of these together enables efficient evaporative cooling from a conservative trap, a procedure which has recently led to the first observations of Bose-Einstein condensation in a dilute atomic vapour. The behaviour of a magneto-optical trap is nominally determined by four independent parameters: the detuning and intensity of the light field, the magnetic field gradient and the number of trapped atoms. A model is presented which incorporates previous treatments into a single description of the trap that encompasses a wide range of its behaviour. This model was tested quantitatively by measuring the temperature of the cloud and its spatial distribution as a function of the four parameters. The maximum density was found to be limited both by the reabsorption of photons scattered within the cloud and by a reduction of the confining force at small light shifts. The nonlinear variation with position of the restoring force was found to be significant in limiting the number of atoms confined to a high density. A maximum density in phase space (defined as the number of atoms in a box with sides of dimension one thermal de Broglie wavelength) of (1.5 ± 0.5) x 10-5 was observed, with a spatial density of 1.5 x 1011 atoms per cm3. Cold collision losses from a caesium magneto-optical trap have been studied with the purpose of assessing their influence on spatial densities. In contrast to previous measurements of similar quantities, these measurements did not require the use of an ultra-low (< 10-10 Torr) background vapour pressure. The dependence of the cold collision loss coefficient β on the trapping intensity was measured to permit identification of the different cold collision processes. The largest loss rates observed were those due to hyperfine structure-changing collisions, with a coefficient β = (2±1) x 10-10cm3s-1. A study is presented of a modified magneto-optical trap in which a fraction of the population is shelved into a hyperfine level that does not interact with the trapping light. In this so-called "dark" magneto-optical trap, improved densities of nearly 1012cm-3 have been previously reported for sodium. The application of the technique to caesium is not straightforward due to the larger excited state hyperfine splittings. A simple theory for caesium is presented and its main predictions verified by measurements of density, number and temperature. A density of nearly 1012cm,-3 was indeed obtained but at a temperature substantially higher than in the conventional magneto-optical trap.
We produced a slow atomic beam of rubidium with a low-velocity intense source (LVIS) and loaded a magneto-optical trap (MOT) in a connected chamber with the beam. The hole for the atomic beam allowed us to keep the MOT chamber at a much lower pressure than that of the LVIS. The LVIS produced an atomic beam with a continuous flux of (4.3±0.4)×109/s and an average velocity of only 11±3 m/s. The MOT captured the atoms with 67±15% efficiency, which was limited by the atomic-beam divergence. The MOT held 6×109 atoms with a typical trap-confinement time of 8 s.