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Laser Physics

Optical levitation-associated atomic loading in a dipole trap

Guosheng Feng, Yuqing Li, Jizhou Wu, V. B. Sovkov, Jie Ma, Liantuan Xiao, Suotang Jia

Published Feb 12, 2019• 1 citation
Publisher

Abstract

We demonstrate a robust method for the efficient optically levitated loading of ultracold Cs atoms in a crossed dipole trap. When preparing a large number of atomic samples, a large-volume crossed dipole trap is required to form a shallow but very efficient loading potential. The scattering force coming from a red near-off-resonance laser is utilized to precisely compensate for the destructive gravitational force of the atoms, making it a promising method for loading and trapping ∼ atoms in a pure optical trap. The optimum levitation laser intensity is ∼177 W cm −2 with a detuning of −25.0 GHz. The dependence of the variation in the number of atoms loaded and trapped in the optically levitated dipole trap on the intensity and detuning of the levitated laser is in good agreement with theoretical predictions.

DOI
10.1088/1555-6611/aafd22

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Acta Physica Sinica•Jan 1, 2016•0 citations
We demonstrate a robust method for the efficient optically levitated loading of ultracold Cs atoms in a crossed dipole trap. When preparing a large number of atomic samples, a large-volume crossed dipole trap is required to form a shallow but very efficient loading potential. The scattering force coming from a red near-off-resonance laser is utilized to precisely compensate for the destructive gravitational force of the atoms, making it a promising method for loading and trapping ∼ atoms in a pure optical trap. The optimum levitation laser intensity is ∼177 W cm −2 with a detuning of −25.0 GHz. The dependence of the variation in the number of atoms loaded and trapped in the optically levitated dipole trap on the intensity and detuning of the levitated laser is in good agreement with theoretical predictions.
We report a detailed study of effective magnetically levitated loading of cold atoms in a crossed dipole trap: an appropriate magnetic field gradient precisely compensates for the destructive gravitational force of the atoms and an additional bias field simultaneously eliminates the antitrapping potential induced by the magnetic field gradient. The magnetic levitation is required for a large-volume crossed dipole trap to form a shallow but very effective loading potential, making it a promising method for loading and trapping more cold atoms. For cold cesium atoms in the F=3,mF=3F=3, mF=3F=3,mF=3 state prepared by three-dimensional degenerated Raman sideband cooling, a large number of atoms $\ensuremath{\sim}3.2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ have been loaded into a large-volume crossed dipole trap with the help of the magnetic levitation technique. The dependence of the number of atoms loaded and trapped in the dipole trap on the magnetic field gradient and bias field, respectively, is in good agreement with the theoretical analysis. The optimum magnetic field gradient of 31.13 G/cm matches the theoretical value of 31.3 G/cm well. This method can be used to obtain more cold atoms or a large number of Bose-Einstein condensation atoms for many atomic species in high-field seeking states.
Abstract We presented a comprehensive study of optimized loading of ultracold Cs atoms in a magnetic levitated crossed dipole trap. Moreover, we analyzed the optimized experimental parameters of the dipole laser for the opening time and the power intensity prior to switching on the magnetic field which formed the magnetically levitated dipole trap. The number of atoms as the function of sweep time from distinct laser power intensity to a fixed one is measured. And beyond that, the variation of atoms with the dipole laser power intensity per beam was studied experimentally ranging from 0 W cm−2 to a fixed one for loading the dipole trap, and the number of atoms can be increased up to a maximum of ∼1.67 × 106. Our research demonstrates that the ‘pre-loading process’ plays a crucial role in enhancing the loading efficiency of an optical dipole trap.
We study the excessive levitation effect in the magnetically levitated loading process of ultracold Cs atoms into a large-volume crossed optical dipole trap. We analyze the motion of atoms with a non-zero combined gravito-magnetic force during the loading, where the magnetically levitated force catches up with and surpasses the gravity. We present the theoretical variations of both acceleration and velocity with levitation time and magnetic field gradient. We measure the evolution of the number of trapped atoms with the excessive levitation time at different magnetic field gradients. The dependence of the number of atoms on the magnetic field gradient is also measured for different excessive levitation times. The theoretical analysis shows reasonable agreement with the experimental results. Our investigation illustrates that the excessive levitation can be used to reduce the heating effect of atoms in the magnetically levitated loading process, and to improve the loading rate of a large-volume optical dipole trap.
We report a detailed study of magnetically levitated loading of ultracold 133Cs atoms in a dimple trap. The atomic sample was produced in a combined red-detuned optical dipole trap and dimple trap formed by two small waist beams crossing a horizontal plane. The magnetic levitation for the 133Cs atoms forms an effective potential for a large number of atoms in a high spatial density. Dependence of the number of atoms loaded and trapped in the dimple trap on the magnetic field gradient and bias field is in good agreement with the theoretical analysis. This method has been widely used to obtain the Bose–Einstein condensation atoms for many atomic species.
We report on the stable effective trapping of cold rubidium atoms by a method for a single optical trap in the near-optical resonant regime. An optical trap with the near-optical resonance condition consists of not only the dipole but also the radiative forces, while a trap using a far-off resonance dominates only the dipole force. We measure the spatial behaviors of the center-of-mass positions and the loading efficiencies of the trapped atoms in the near-optical resonant trap, by changing the detuning over the range of $\ensuremath{-}0.373$ to $\ensuremath{-}2.23$ THz from the D2{\mathrm{D}}_{2}D2​ resonance. The time dependence of the spatial behavior in the trap indicates that the suitable positions for stably holding atoms are altered because the equilibrium condition between the optical dipole and the radiative forces depends on the trap laser detuning. The stable position, which is not a primary position of focus in the Gaussian beam optics, depends only on the laser detuning due to the change in the radiative force, while it is independent of the change in the laser intensity, which results in a balance between the radiative and dipole forces.
Optical trapping techniques and the ability to tune the atomic interactions both have made the unprecedented progress in the quantum gas research field. The major advantage of the optical trap is that the atoms are likely to be trapped at various sub-levels of the electronic ground state and the interaction strength can be controlled by Feshbach resonance. Optical trapping methods in combination with magnetic tuning of the scattering properties directly lead to the experimental achievements of Bose-Einstein condensation (BEC) of Cesium, which at first failed by using magnetic trapping approaches due to the large inelastic collision rate. The rapid loss of cesium atoms due to the inelastic two-body collisions greatly suppresses the efficient evaporative cooling to obtain a condensate. For optical production of cesium atomic BEC, it is necessary to prepare a large number of Cs atoms at specified state in an optical trap for condensation, especially for an efficient forced evaporation cooling. In this paper, we demonstrate our research on enhancing the loading rate of the atoms by using a dimple trap combined with a large-volume optical dipole trap (reservoir trap). In our work, the cold cesium atoms are prepared by a three-dimensional degenerated Raman sideband cooling, and then loaded into a large-volume crossed dipole trap by using the magnetic levitation technique. Effective load of the dimple optical trap is realized by superposing the small-volume dimple trap on the center of the largevolume optical trap. The theoretical analyses are performed for the magnetically levitated large-volume crossed dipole trap in variable magnetic field gradients and uniform bias fields. Optimal experimental values are acquired accordingly. The combined potential curve of the dimple trap, which is superimposed on the magnetically levitated large-volume dipole trap, is also given. The loading of precooled atoms from Raman sideband cooling into the magnetically levitated large-volume optical trap is measured in variable magnetic field gradients and uniform bias fields. Different loading results of the dimple trap are investigated, including direct loading after Raman sideband cooling, the large-volume optical trap and the magnetically levitated large-volume dipole trap without anti-trapping potential. Comparatively, the atomic number density is enhanced by a factor of ~15 by loading the atomic sample from the magnetically levitated large-volume dipole trap into the dimple optical trap. The experimental results lay a sound basis for the further cooling and densifying the atomic cloud through the evaporating cooling stage. This method can be used to obtain more cold atoms or a large number of Bose-Einstein condensation atoms for atomic species with large atom mass.