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Japanese Journal of Applied Physics

Observation of Double Stable Clouds of Cold Atoms in Magneto-Optical Traps

Vanderlei Salvador Bagnato, N. P. Bigelow, Luís Gustavo Marcassa, Sérgio Carlos Zílio

Published Sep 1, 1996• 1 citation
Publisher

Abstract

We present the observation of double stable clouds of cold trapped atoms in a magneto-optical trap and its explanation with a simple model based on radiative forces. This structure occurs for a specific misalignment of the trapping laser beams whose Gaussian profiles introduce an extra term in the equation of motion for the trapped atom. We were able to obtain simple analytical expressions for the trap potential and for the separation of the clouds, and carried out a numerical calculation which agrees well with the experimental observations.

DOI
10.1143/jjap.35.4664

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We present the observation of double stable clouds of cold trapped atoms in a magneto-optical trap and its explanation with a simple model based on radiative forces. This structure occurs for a specific misalignment of the trapping laser beams whose Gaussian profiles introduce an extra term in the equation of motion for the trapped atom. We were able to obtain simple analytical expressions for the trap potential and for the separation of the clouds, and carried out a numerical calculation which agrees well with the experimental observations.
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We investigated how the size of ring-shaped clouds of cold, magneto-optically trapped sodium atoms depends on the trapping parameters: magnetic-field gradient and laser intensity, detuning, and misalignment. The experiments were carried out in the low-density regime (N < 106 atoms), and the ring radius was shown to be independent of the number of trapped atoms. The results are explained with a simple model including a coordinate-dependent vortex force and the usual magneto-optical trap forces.
In this work, we present a theoretical study of the origin of the spatial structures in the conventional magneto-optical trap by using a numerical model based on the solutions of the equations of motion of trapped atoms, in the limit where the radiation trapping force is not relevant. Some results relating to the comparison between the experimental and theoretical data are reported. Also, we show that this same analysis can be employed to describe the spatial structures formation in atomic beams.
A magneto-optical trap of cesium atoms was generated by applying a circularly polarized cooling laser beam onto a reflective two-dimensional diffraction grating with an aperture and by retroreflecting the incident beam passing through the aperture while reversing the circular polarization. The cooling laser beams comprised the incident, retroreflected, and four diagonally diffracted beams at an angle of 50° with respect to the normal direction of the grating surface. The intensity of the retroreflected beam was carefully adjusted to balance the radiation forces acting on the atoms. A significant number of cold atoms (7.0 × 106) were captured, in spite of the difficulty in a magneto-optical trap of cesium atoms, using nonorthogonal cooling beams owing to the high nuclear spin. The significance of the retroreflected beam in the trapping process was highlighted when the intensity of the retroreflected beam was reduced, resulting in the absence of trapped atoms. Notably, the cold atom cloud was generated near the edge of the region, where all the cooling beams were overlapped. The phenomenon is explained by the numerical calculations of the radiation forces considering all the Zeeman sublevels in the cooling transition and the Gaussian intensity profile of the incident beam.
A single-particle kinetic theory of the magneto-optical trap (MOT) for cold atoms is developed for a multilevel dipole interaction scheme. The theory analyzes the one-dimensional motion of (3+5)(3+5)(3+5)-level atoms in an inhomogeneous magnetic field and in the field of two counterpropagating circularly polarized laser waves. It is shown that in multilevel interaction schemes two-photon and generally even-order multiphoton processes are responsible for the formation of the narrow velocity and spatial structures in the radiation pressure force. The structures are located at zero velocity and near the origin of the trap. It is found that in the case of a (3+5)(3+5)(3+5)-level atom, single- and two-photon processes produce the two-component structure of the MOT potential well. Near the bottom of a conventional spatially broad potential well caused by one-photon processes, there is a much narrower well caused by two-photon processes. The double-structure MOT potential well is shown to be responsible for the two-component spatial distribution of the trapped atomic cloud. It is proposed that tight confinement of atoms near the narrow bottom of the MOT potential well can be used for the creation of a pointlike source of cold atoms for various applications in atom optics.