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Journal of Physics B Atomic Molecular and Optical Physics

Ultracold collisions of mixed atoms in optical dipole trap loaded from a dark magneto-optical trap

Yanting Zhao, Ting Gong, Zhonghao Li, Zhonghua Ji, Xiang Zhang, Liantuan Xiao, Suotang Jia

Published Aug 31, 2017• 0 citations
Publisher

Abstract

Abstract We study the cold collisions of mixed atoms in an optical dipole trap (ODT), which are loaded from a dark magneto-optical trap (MOT). A comprehensive, phenomenological rate equation is presented to derive the ultracold homonuclear and heteronuclear collision rates in loading and holding procedures. Our results show that the cold atoms in the dark MOT can provide a much better stable, initial atomic sample than MOT. The dependence of the heteronuclear collision rate on the trap depth is attributed to the hyperfine-changing collision by the ODT laser with a broad linewidth. The processes of deriving the collision rate are also universal for other kinds of atoms or even molecules.

DOI
10.1088/1361-6455/aa8964

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Abstract We study the cold collisions of mixed atoms in an optical dipole trap (ODT), which are loaded from a dark magneto-optical trap (MOT). A comprehensive, phenomenological rate equation is presented to derive the ultracold homonuclear and heteronuclear collision rates in loading and holding procedures. Our results show that the cold atoms in the dark MOT can provide a much better stable, initial atomic sample than MOT. The dependence of the heteronuclear collision rate on the trap depth is attributed to the hyperfine-changing collision by the ODT laser with a broad linewidth. The processes of deriving the collision rate are also universal for other kinds of atoms or even molecules.
In this work, we load a 1070 nm crossed optical dipole trap (ODT) from a 133Cs dark spontaneous emission optical trap. Obvious improvements in the loading rate and atomic lifetime are demonstrated, comparing with the values of the atoms which are loaded directly from a normal magneto-optical trap. We analyze the atomic number as a function of time in the ODT’s loading and holding processes based on the constant density and the constant volume approximations, respectively. We also demonstrate that the atomic temperature in holding process is related to the atomic collision, which consists of two-body collision and background gas induced collision. Finally the relationship between atomic collision and optical trap depth is measured and analyzed. The atomic loading way we present can be easily expanded to other species to obtain an atomic sample with large number and high density.
The production and research of ultracold heteronuclear molecules have aroused the great interest recently. On the one hand, these molecules are extremely popular in experiments for exploring the collision dynamic behaviors in threshold, photoassociative spectrum and strong dipole-dipole interactions. On the other hand, ultracold polar molecules populated at deeply bound levels in the singlet ground state are the right candidates to investigate quantum memories for quantum simulation, and to study strongly interacting quantum degenerate gases. The precise knowledge of cold collision processes between two different types of alkali atoms is necessary for understanding and utilizing ultracold heteronuclear molecules, sympathetic cooling, and thus formation of two species BEC. The goal of the present investigation is to study the collisions between ultracold sodium atoms and cesium atoms. We systematically demonstrate simultaneously trapping ultracold sodium and cesium atoms in a dual-species magneto-optical trap (MOT). The sodium atom trap loss rate coefficient Na-Cs is measured as a function of Na trapping laser intensity. At low intensities, the trap loss is dominated by ground-state hyperfine-changing collisions, while at high intensities, collisions involving excited atoms are more important. A strong interspecies collision-induced loss for Na atoms in the MOT is observed. In order to obtain the trap loss coefficient Na-Cs, we proceed in two steps. First, the Cs repumping laser is blocked to avoid the formation of ultraold Cs atoms. The loading process of Na atoms is recorded when the Cs trapping laser is on. Second, the loading curves of the Na atoms are obtained as Cs atoms are present with the repumping laser beams. The total losses PNa and PNa' are acquired by fitting the two loading curves of trapped Na atoms. Thus, the trap loss coefficient Na-Cs can be derived from the difference between total losses PNa and PNa' divided by the density of the Cs atoms. The coefficient Na-Cs decreases in a range of 5-10mW/cm2, which originates from the hyperfine-state changing (HFC) collision. A Doppler model is used to calculate the Na atom trap depth, in that the atom trap depth and exoergic energy determine the behavior of the collisional trap loss rate coefficient. The three corresponding calculated critical intensities of Na trapping laser are 7.98, 14.82, 16.2 mW/cm2 respectively in the Na-Cs HFC collision process. The first calculated critical intensity value agrees well with the experimental result. Our work provides a valuable insight into HFC collision between Na and Cs atoms and also paves the way for the production of ultracold NaCs molecules using Photoassociation (PA) technology. Furthermore, the experimental results lay a great basis for the obtainments of high sensitive heteronuclear NaCs molecular PA spectrum and the creation of deeply bound ground state molecules.
In this paper, ultracold atoms and molecules in a dark magneto-optical trap (MOT) are studied via depumping the cesium cold atoms into the dark hyperfine ground state. The collision rate is reduced to 0.45 s −1 and the density of the atoms is increased to 5.6 × 10 11 cm −3 when the fractional population of the atoms in the bright hyperfine ground state is as low as 0.15. The vibrational spectra of the ultracold cesium molecules are also studied in a standard MOT and in a dark MOT separately. The experimental results are analyzed by using the perturbative quantum approach.
In this paper,ultracold atoms and molecules in a dark magneto-optical trap(MOT) are studied via depumping the cesium cold atoms into the dark hyperfine ground state.The collision rate is reduced to 0.45s-1 and the density of the atoms is increased to 5.6×1011cm-3 when the fractional population of the atoms in the bright hyperfine ground state is as low as 0.15.The vibrational spectra of the ultracold cesium molecules are also studied in a standard MOT and in a dark MOT separately.The experimental results are analyzed by using the perturbative quantum approach.
We report the first realization of an optical dipole trap (ODT) for ultracold mercury atoms, overcoming the challenge posed by the exceptionally low polarizability of Hg and the resulting weak optical trapping potential. We demonstrate the transfer of six naturally abundant Hg isotopes from a magneto-optical trap (MOT) into the ODT and characterize its loading dynamics, trap depth, and lifetime. Confinement of dense samples of ultracold Hg opens new opportunities for studies of isotope-dependent collisions and photoassociation and provides a route toward quantum degeneracy.
We present a detailed experimental study of the physics involved in transferring atoms from a magneto-optical trap (MOT) to an optical dipole trap. The loading is a dynamical process determined by a loading rate and a density dependent loss rate. The loading rate depends on cooling and the flux of atoms into the trapping volume, and the loss rate is due to excited state collisions induced by the MOT light fields. From this study we found ways to optimize the loading of the optical dipole trap. Key ingredients for maximum loading are found to be a reduction of the hyperfine repump intensity, increased detuning of the MOT light, and a displacement of the optical dipole trap center with respect to the MOT. A factor of 2 increase in the number of loaded atoms is demonstrated by using a hyperfine repump beam with a shadow in it. In this way we load $8\ifmmode\times\else\texttimes\fi{}{10}^{6}{}^{85}\mathrm{Rb}$ atoms into a 1 mK deep optical dipole trap with a waist of $58 \ensuremath{\mu}\mathrm{m},$ which is 40% of the atoms initially trapped in the MOT.