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arXiv:0801.2791 · Optics Express

Versatile two-dimensional potentials for ultra-cold atoms

S. K. Schnelle, Erik van Ooijen, Matthew J. Davis, N. R. Heckenberg, Halina H. Rubinsztein-Dunlop

Published Jan 1, 2008• 70 citations
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Abstract

We propose and investigate a technique for generating smooth two-dimensional potentials for ultra-cold atoms based on the rapid scanning of a far-detuned laser beam using a two-dimensional acousto-optical modulator (AOM). We demonstrate the implementation of a feed-forward mechanism for fast and accurate control of the spatial intensity of the laser beam, resulting in improved homogeneity for the atom trap. This technique could be used to generate a smooth toroidal trap that would be useful for static and dynamic experiments on superfluidity and persistent currents with ultra-cold atoms.

DOI
10.1364/oe.16.001405

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We propose and investigate a technique for generating smooth two-dimensional potentials for ultra-cold atoms based on the rapid scanning of a far-detuned laser beam using a two-dimensional acousto-optical modulator (AOM). We demonstrate the implementation of a feed-forward mechanism for fast and accurate control of the spatial intensity of the laser beam, resulting in improved homogeneity for the atom trap. This technique could be used to generate a smooth toroidal trap that would be useful for static and dynamic experiments on superfluidity and persistent currents with ultra-cold atoms.
The loading of ultra-cold atoms into two-dimensional, time-averaged optical potentials was demonstrated. The time-averaging scheme allows us to make arbitrary and two dimensional potentials, where the potential shape and depth can be dynamically controlled. Traps are produced by rapidly scanning a red-detuned laser focus in any desired pattern using a two-dimensional acousto-optical modulator (AOM). Smooth optical potentials are created when the scanning speed is much higher than any trapping frequency in system, resulting in a time-averaged dipole potential for the atoms. The depth of the potential can be controlled at any time and place by modulating the amplitude of the applied radio frequency to the AOM.
Summary form only given. A rapidly scanning far-detuned laser beam can generate a time-averaged optical dipole potential provided that the scan speed is much faster then any characteristic atomic motional time scale. The slow motional time scales of ultra cold atoms enables the use of even tightly focused scanning beams with readily attainable scan speeds of few hundred kHz. This concept offers the possibility to form nearly arbitrary effective optical potentials and also to change them in real time. It was recently used for adiabatic compression of rubidium atoms in a single-beam dark optical trap up to densities of 5/spl times/10/sup 13/ cm/sup -3/. Here we report on three new atom-optics applications based on rapidly scanning beams.
This thesis presents work on Bose-Einstein condensates in non-harmonic optical potentials. First, a new trap design developed at the University of Queensland is presented that allows the creation of nearly arbitrary two-dimensional potential landscapes by spatially scanning a far red-detuned laser beam using a two-dimensional acousto-optic modulator. In conjunction with a feed-forward technique this trap is capable of producing optical traps which have the necessary stability to be used in ultra-cold atom research. Different geometries are presented. In particular toroidal trap geometries are discussed which are interesting because they offer the possibility for a multiply connected Bose-Einstein condensate. The trap also offers the possibility of dynamic potentials which have been employed to measure the critical velocity of superfluidity in Bose-Einstein condensates. Secondly, measurements on condensation dynamics are presented which use an optical dimple potential superimposed upon a harmonic magnetic trap. In the experiments the dimple potential is ramped on slowly or turned on suddenly for a range of dimple depths and widths and the condensate fraction and temperature are measured as a function of hold times. The measurements taken are compared to equilibrium thermodynamics and quantum kinetic theory and for the first time a quantitative model is presented that allows correct prediction of condensate fractions and temperatures in good agreement with the measurements. Also presented are first results in condensate rethermalization experiments where a dimple potential is suddenly switched off and the condensate left to rethermalize in a much wider magnetic potential. The measurements show a sudden breakdown in condensate fraction followed by a subsequent recovery. So far no theory exists to properly interpret the results as quantum kinetic theory cannot be applied to the problem. Lastly, progress on experiments to measure the critical velocity of superfluidity in Bose-Einstein condensates is reported. Calculations have been carried out that show the creation of solitons and vortices as mechanisms to dissipate energy leading to a breakdown of superfluid behaviour at velocities lower than that predicted by Landau’s theory of superfluidity, the first theory to describe superfluidity. Progress on an experiment is presented that uses the scanning beam trap developed as part of this project to create a line potential with a moving barrier to measure the critical velocity and show solitons and vortices as the mechanisms for energy dissipation as seen in the calculations.
Since the first Bose-Einstein Condensate (BEC) was demonstrated in a dilute atomic gas [1], rotating BECs have been of interest to researchers due to their quantized circulation. We propose a new method of creating controlled vortices in a BEC. Atoms are held in the focus of a scanning beam that rapidly scans to produce a time-averaged potential [2]. We have made theoretical calculations to measure the heating caused by a scanning beam trap and to find the regime where a scanning beam produces a time-averaged potential. The scanning beam splits the BEC into four parts then imprints a relative phase on each part such that, when the parts recombine, a vortex forms. We have simulated such a system using the Gross-Pitaevskii equation and have demonstrated the laser scanning pattern using a two-dimensional acousto-optic modulator (AOM). The optics for the scanning beam trap have been designed and the experimental trap dimensions are used in our simulations. To carry out the experimental part of this project, we are currently building and testing an all-optical BEC apparatus: we have made a magneto-optical trap (MOT) which cools the 87Rb atomsto 30uK. We are currently setting up an optical dipole trap to perform the final stages of cooling.