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Journal of Physics B Atomic Molecular and Optical Physics

Atomic beam focusing with a curved magnetic mirror

H. Merimeche

Published Sep 4, 2006• 9 citations
Publisher

Abstract

Reflective atom optics has been experimentally realized with a continuous flux of cold atoms by using a magnetized videotape. We have studied the behaviour of a slow atomic beam with a velocity of 20 m s(-1) incident at different angles on the surface of a curved magnetic mirror. Focusing of the beam has been demonstrated and this induces an amplification up to a factor 3 of the fluorescence emitted by the atoms around the focal point. We deduced the shape of our reflector from the focal length measurements and show that the result is completely consistent with those obtained from the reflection of a red diode laser beam on the curved mirror.

DOI
10.1088/0953-4075/39/18/002

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Laser cooling methods provide the possibility to obtain very well collimated and slow atomic beams. These intense sources have attracted new interest in the manipulation of atomic trajectories. Atom optical schemes based on the optical dipole force are very attractive, for example in atom lithography where atomic beams are focused by standing wave light elds, to create periodic nanostructures. However, in applications where more complex structures are desirable or decoherence due to spontaneous emission has to be avoided, magnetic atom optical components are more attractive. For example, at and curved atomic mirrors with high reflectivity and low surface rough ness can be built from periodically magnetized videotapes. First, with a collimated and polarized cesium atomic beam from a Zeeman-slower apparatus, we have investigated the deflection of the atomic beam by a sinusoidally magnetized videotape with a period length of 30 μm. From these measurements we have determined the magnetic eld strength at the surface to be 0.05 Tesla. The reflectivity has been found to be consistent with 100%. The very large eld gradients produced by the tape can be exploited to guide atoms on curved trajectories with small radii of curvature. With a radius of 9.5 cm we have obtained a maximum deflection angle of 25° which is limited by technical constraint. Second, we have built a setup based on a 2D-MOT for generating a slow atomic beam with a velocity of 20 m/s. After leaving the optical collimation and the optical polarization stages, the atomic beam is reflected after one bounce onto the videotape up to angles as large as 7°. We exploited the natural curvature of our videotape and we demonstrated the focusing of the atomic beam after one and two bounces depending on the curvature of the videotape. The focal length and the angles of deflection enable to deduce the local radii of curvature. We have confronted our results by testing the focusing of the videotape with a red laser and by performing a numerical simulation which prove the consistency with the atomic beam experiments.

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 1, 1997•3 citations
Reflective atom optics has been experimentally realized with a continuous flux of cold atoms by using a magnetized videotape. We have studied the behaviour of a slow atomic beam with a velocity of 20 m s(-1) incident at different angles on the surface of a curved magnetic mirror. Focusing of the beam has been demonstrated and this induces an amplification up to a factor 3 of the fluorescence emitted by the atoms around the focal point. We deduced the shape of our reflector from the focal length measurements and show that the result is completely consistent with those obtained from the reflection of a red diode laser beam on the curved mirror.
Laser cooling methods provide the possibility to obtain very well collimated and slow atomic beams. These intense sources have attracted new interest in the manipulation of atomic trajectories. Atom optical schemes based on the optical dipole force are very attractive, for example in atom lithography where atomic beams are focused by standing wave light elds, to create periodic nanostructures. However, in applications where more complex structures are desirable or decoherence due to spontaneous emission has to be avoided, magnetic atom optical components are more attractive. For example, at and curved atomic mirrors with high reflectivity and low surface rough ness can be built from periodically magnetized videotapes. First, with a collimated and polarized cesium atomic beam from a Zeeman-slower apparatus, we have investigated the deflection of the atomic beam by a sinusoidally magnetized videotape with a period length of 30 μm. From these measurements we have determined the magnetic eld strength at the surface to be 0.05 Tesla. The reflectivity has been found to be consistent with 100%. The very large eld gradients produced by the tape can be exploited to guide atoms on curved trajectories with small radii of curvature. With a radius of 9.5 cm we have obtained a maximum deflection angle of 25° which is limited by technical constraint. Second, we have built a setup based on a 2D-MOT for generating a slow atomic beam with a velocity of 20 m/s. After leaving the optical collimation and the optical polarization stages, the atomic beam is reflected after one bounce onto the videotape up to angles as large as 7°. We exploited the natural curvature of our videotape and we demonstrated the focusing of the atomic beam after one and two bounces depending on the curvature of the videotape. The focal length and the angles of deflection enable to deduce the local radii of curvature. We have confronted our results by testing the focusing of the videotape with a red laser and by performing a numerical simulation which prove the consistency with the atomic beam experiments.
Videotape with a sinusoidal magnetization of $31 \ensuremath{\mu}\mathrm{m}$ wavelength is used to reflect Cs atoms with unit reflectivity in a 75 m/s atomic beam. The atoms serve as a probe, allowing us to measure the magnetic field at the surface. A technique is presented for mounting the videotape so that its surface can be curved to a specific shape or made flexible. We show that such a reflector provides high-quality grazing-incidence atom optics and we demonstrate deflections as large as $23\ifmmode^\circ\else\textdegree\fi{}$ in a whispering-gallery geometry.
We have demonstrated that cold atoms can be retroflected from a ferromagnetic surface by the Stern-Gerlach effect. When the surface is magnetized periodically, the reflectivity for suitably polarized atoms is $(94\ifmmode\pm\else\textpm\fi{}8)%$ and the reflection is specular. A demagnetized surface is also highly reflecting but the reflections are diffuse. These magnetic processes are of interest for atom optics because they permit the manipulation of cold atoms without the use of laser beams. In our experiments, Rb atoms released from a magneto-optic trap (MOT) fall under gravity until they are reflected, after which they are recaptured in the MOT. Multiple bounces of these atoms have been studied for times up to 600 ms.
We have obtained and characterized a cold atomic beam produced by a simple conical atom funnel, which consists of a conical hollow mirror (axicon) with a small hole at the apex and a single incident laser beam. Atoms trapped in the axicon mirror are ejected by the radiation-pressure imbalance along the extraction column. The atomic beam has a longitudinal velocity in the range of 4 to 6 m/s, a divergence of 150 mrad, and a maximum instantaneous flux of $2.2\ifmmode\times\else\texttimes\fi{}{10}^{10} \mathrm{atoms}/\mathrm{s}$ in a pulsed operation. The experimental results are in quantitative agreement with the numerical simulations. A continuous atomic beam is also possible with an apex hole smaller than the trapped atomic cloud.
Multiple bounces of cold rubidium atoms have been observed for times up to one second in a trap formed by gravity and a 2 cm-diameter spherical mirror made from a sinusoidally magnetized floppy disk. We have studied the dynamics of the atoms bouncing in this trap from several different heights up to 40.5 mm and we conclude that the atoms are reflected specularly and with reflectivity 1.01(3). Slight roughness of the mirror is caused by harmonics in the magnetization of the surface and by discontinuities at the boundaries between recorded tracks. As the next step in this atom optics program we propose using a magnetic mirror to create a 2D atomic gas. We discuss how cold atoms can be loaded into the ground state of a static magnetic potential well that exists above the surface of the mirror as a consequence of the intermediate-field Zeeman effect.