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arXiv:physics/0204026 · Physical Review A

Cold-atom gas at very high densities in an optical surface microtrap

M. Hammes, D. Rychtarik, Hanns‐Christoph Nägerl, Rudolf Grimm

Published Nov 14, 2002• 30 citations
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Abstract

An optical microtrap is realized on a dielectric surface by crossing a tightly focused laser beam with a horizontal evanescent-wave atom mirror. The nondissipative trap is loaded with $\ensuremath{\sim}{10}^{5}$ cesium atoms through elastic collisions from a cold reservoir provided by a large-volume optical surface trap. With an observed 300-fold local increase of the atomic number density approaching ${10}^{14}{\mathrm{cm}}^{\ensuremath{-}3},$ unprecedented conditions of cold atoms close to a surface are realized.

DOI
10.1103/physreva.66.051401

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An optical microtrap is realized on a dielectric surface by crossing a tightly focused laser beam with a horizontal evanescent-wave atom mirror. The nondissipative trap is loaded with \ensuremath∼105\ensuremath{\sim}{10}^{5}\ensuremath∼105 cesium atoms through elastic collisions from a cold reservoir provided by a large-volume optical surface trap. With an observed 300-fold local increase of the atomic number density approaching 1014cm\ensuremath−3,{10}^{14}{\mathrm{cm}}^{\ensuremath{-}3},1014cm\ensuremath−3, unprecedented conditions of cold atoms close to a surface are realized.
During the past two decades the dramatic developments in cooling and trapping of gaseous atomic samples has produced a variety of techniques to manipulate the external and internal atomic states with electromagnetic fields. The research efforts so far have mainly focused on studying the fundamental aspects of modern quantum physics, although numerous practical applications, e.g., in integrated matter-wave optics and quantum information processing, are expected to soon be added to the already established atom-optics products, such as atomic clocks and acceleration sensors. One of the most promising foundations for the practical applications of Bose-Einstein condensates (BECs) and coherent matter waves is the technology based on the creation of microscopic atom traps on the surface of a solid substrate. This trapping technique has recently been used to successfully control the motion of microscopic atomic samples and to significantly simplify the creation of BEC.\n\nThe main research topic of this thesis is the development and design of surface-mounted atom traps on transparent dielectric substrates. Such traps could provide an extra flexibility and stability for the experiments, since they would allow unimpeded control of atoms with laser light and provide reduction of magnetic-field fluctuations associated with the thermal motion of free electrons in the substrate material. The thesis describes several novel approaches to the creation of such surface traps by superimposing repulsive evanescent optical waves with strongly localized magnetic or electric fields. These fields can be produced by either conductive or permanently magnetized, optically transparent patterns imprinted in a thin layer on a transparent dielectric substrate. The evanescent wave can also be used to cool the atoms in a gravitational field before loading them into the microtrap. The lateral confinement of the atoms on the evanescent wave can be realized with a thin-walled hollow laser beam, the creation of which is demonstrated in the thesis. The thesis also describes certain general aspects concerning evanescent-wave cooling. In particular, the influence that multiple reabsorption of resonance-frequency photons in a cloud of evanescent-wave cooled atoms has on the cooling efficiency is investigated. Also, a theoretical model based on classical statistical mechanics and thermodynamics is introduced to show how a microtrap on an evanescent-wave mirror can be used to decrease the temperature, increase the phase-space density, and provide temperature conserving spin-polarization of the atoms.
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We report on the creation of a two-dimensional Bose-Einstein condensate of cesium atoms in a gravito-optical surface trap. The condensate is produced a few microm above a dielectric surface on an evanescent-wave atom mirror. After evaporative cooling by all-optical means, expansion measurements for the tightly confined vertical motion show energies well below the vibrational energy quantum. The presence of a condensate is observed in two independent ways by a magnetically induced collapse at negative scattering length and by measurements of the horizontal expansion.