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Chinese Physics Letters

Realization of High Optical Density Rb Magneto-optical Trap

Yanxu Han, Liu Y, Chunhong Zhang, Shujing Li, Wang Hai

Published Feb 1, 2009• 3 citations
Publisher

Abstract

We report experimental demonstration of a magneto-optical trap (MOT) of Rb atoms with a high optical density. With 2.2-cm-diameter cooling laser beams, we achieve an optical density of nearly 11 for about 2.6 × 10 10 trapped Rb atoms with the beam intensity of about 6.6 mW/cm per beam. The temperature of the cold atoms is about 250 μK. Furthermore, by ramping the magnetic field gradient from 8 G/cm to about 20 G/cm, the atomic cloud in the MOT is compressed and the optical density is up to 16.

DOI
10.1088/0256-307x/26/2/023201

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We report experimental demonstration of a magneto-optical trap (MOT) of Rb atoms with a high optical density. With 2.2-cm-diameter cooling laser beams, we achieve an optical density of nearly 11 for about 2.6 × 10 10 trapped Rb atoms with the beam intensity of about 6.6 mW/cm per beam. The temperature of the cold atoms is about 250 μK. Furthermore, by ramping the magnetic field gradient from 8 G/cm to about 20 G/cm, the atomic cloud in the MOT is compressed and the optical density is up to 16.
We have built an apparatus for setting up a double magneto-optical trap (double MOT) which allows more than 10 Rb atoms to be loaded in 15 seconds. This compact system consists mainly of a stainless steel chamber and a homemade glass cell. The two chambers are spatially separated by a small diameter transfer tube for differential pumping. The stainless steel chamber is maintained at a pressure of 10 Torr, enabling fast capturing of background Rb atoms. The optically cooled atomic clouds are pushed by a laser beam towards the glass cell which is in a much higher vacuum. Under this condition, the glass cell provides a rather long life time for holding the atoms. After the atoms are loaded in the temporarily bright MOT, a semi-dark type MOT compression is employed to reach a spatial density of 1×10 atoms/cm. The compressed atom clouds are further cooled by polarization gradient cooling to a temperature of 30 μK. The high phase space density cloud is suitable for experiments related to ultracold atoms.
The magneto-optical trap (MOT) serves as the culmination of optical cooling and trapping, where by applying a linear magnetic field to orthogonal, circularly polarized, retroreflecting laser beams, one can produce a dense cloud of very cold atoms, on the order of μK. This work set out to create a magneto-optical trap using only visible light with the 6s 2S1/2 → 7p 2P◦ 3/2 transition of cesium-133, which is far weaker than the transition typically employed for a cesium-133 MOT. This transition alone has yet to be used in isolation to create a MOT, and the purpose of this work is to demonstrate the feasibility of such a MOT. While the creation of a MOT with the setup produced has thus far been unsuccessful, the components are in place for one to be made as a demonstration of optical cooling and trapping that should be visible to the naked eye. Additionally, an experimental setup was created for high precision measurements of the absolute energies of the 7p 2P◦ 1/2 and 7p 2P◦ 3/2 states. The theory behind the MOT and high precision spectroscopy is expounded upon within.
This thesis presents the study of atomic cloud density and temperature in a magneto-optical trap (MOT). The purpose is to find the method in obtaining the densest and coldest cloud the setup can produce. In steady state trap, the highest atomic density possible is that of a cloud in multiple scattering regime where a repulsive force between atoms sets a limit to the density. The number of atoms loaded into the trap is controlled by the trapping beam intensity which also changes the temperature. Therefore the trap density and temperature cannot be controlled separately. The cloud compression is studied as a method of increasing atomic density above what is possible in steady state trap without noticeable influence on the cloud temperature. Cloud compression is also found when the cloud is translated by changing the magnetic field zero-point although in a less predictable fashion than the compression.
 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.
Laser cooled and trapped atoms have been the workhorse of atomic physics for the past four decades. The predominant method has been the highly versatile Magneto-Optical Trap. We describe an alternative laser trap involving a simple geometry of collimated laser beams that provides both a velocity and position dependent restoring force such that a dense cloud of cold atoms is formed. This technique produces similar atom number (>106>10^6>106) and density (101010^{10}1010\,atoms/cm3^{3}3) to the Magneto-Optical Trap, albeit with \emph{no magnetic field}. The beam geometry is compatible with conventional sub-Doppler cooling techniques, allowing the trapped cloud to be cooled to <10 μ< 10~μ<10 μK. We demonstrate the validity and robustness of the trap by capturing 87^{87}87Rb atoms directly from the background vapor and provide a theoretical discussion of the underlying principles. This trap has many unique properties that make it highly suitable for quantum sensing, timing, and computing applications as well as a new tool in fundamental science and metrology.