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Journal of Physics Communications

Comparison between 403 nm and 497 nm repumping schemes for strontium magneto-optical traps

Paulo Hisao Moriya, M. Araujo, Fagner R. Todão, M. Hemmerling, Hans Keßler, R. F. Shiozaki, R Teixeira, Ph. W. Courteille

Published Dec 6, 2018• 21 citations
Publisher

Abstract

The theoretical description of the external degrees of freedom of atoms trapped inside a magnetooptical trap (MOT) often relies on the decoupling of the evolution of the internal and external degrees of freedom.That is possible thanks to much shorter timescales typically associated with the first ones.The electronic structure of alkaline-earth atoms, on the other hand, presents ultra-narrow transitions and metastable states that makes such an approximation invalid in the general case.In this article, we report on a model based on open Bloch equations for the evolution of the number of atoms in a magneto-optical trap.With this model we investigate the loading of the strontium blue magnetooptical trap under different repumping schemes, either directly from a Zeeman slower, or from an atomic reservoir made of atoms in a metastable state trapped in the magnetic quadrupolar field.The fluorescence observed on the strong 461 nm transition is recorded and quantitatively compared with the results from our simulations.The comparison between experimental results and calculations within our model allowed to identify the existence of the decay paths between the upper level of the repumping transition and the dark strontium metastable states, which could not be explained by electric dipole transition rates calculated in the literature.Moreover, our analysis pinpoints the role of the atomic movement in limiting the efficiency of the atomic repumping of the Sr metastable states.

DOI
10.1088/2399-6528/aaf662

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The theoretical description of the external degrees of freedom of atoms trapped inside a magnetooptical trap (MOT) often relies on the decoupling of the evolution of the internal and external degrees of freedom.That is possible thanks to much shorter timescales typically associated with the first ones.The electronic structure of alkaline-earth atoms, on the other hand, presents ultra-narrow transitions and metastable states that makes such an approximation invalid in the general case.In this article, we report on a model based on open Bloch equations for the evolution of the number of atoms in a magneto-optical trap.With this model we investigate the loading of the strontium blue magnetooptical trap under different repumping schemes, either directly from a Zeeman slower, or from an atomic reservoir made of atoms in a metastable state trapped in the magnetic quadrupolar field.The fluorescence observed on the strong 461 nm transition is recorded and quantitatively compared with the results from our simulations.The comparison between experimental results and calculations within our model allowed to identify the existence of the decay paths between the upper level of the repumping transition and the dark strontium metastable states, which could not be explained by electric dipole transition rates calculated in the literature.Moreover, our analysis pinpoints the role of the atomic movement in limiting the efficiency of the atomic repumping of the Sr metastable states.
The theoretical description of the external degrees of freedom of atoms trapped inside a magneto-optical trap (MOT) often relies on the decoupling of the evolution of the internal and external degrees of freedom. That is possible thanks to much shorter timescales typically associated with the first ones. The electronic structure of alkaline-earth atoms, on the other hand, presents ultra-narrow transitions and metastable states that makes such an approximation invalid in the general case. In this article, we report on a model based on open Bloch equations for the evolution of the number of atoms in a magneto-optical trap. With this model we investigate the loading of the strontium blue magneto-optical trap under different repumping schemes, either directly from a Zeeman slower, or from an atomic reservoir made of atoms in a metastable state trapped in the magnetic quadrupolar field. The fluorescence observed on the strong 461~nm transition is recorded and quantitatively compared with the results from our simulations. The comparison between experimental results and calculations within our model allowed to identify the existence of the decay paths between the upper level of the repumping transition and the dark strontium metastable states, which could not be explained by electric dipole transition rates calculated in the literature. Moreover, our analysis pinpoints the role of the atomic movement in limiting the efficiency of the atomic repumping of the Sr metastable states.
We demonstrate enhanced loading of strontium atoms into a magneto-optical trap using a repumping scheme from the metastable state via the doubly excited state $5s5p{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{2}\ensuremath{\rightarrow}5{p}^{2}{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{2}$ at 481nm481\phantom{\rule{3.33333pt}{0ex}}\mathrm{nm}481nm. The number of trapped atoms is increased by an order of magnitude. The frequency and intensity dependence of the atom number enhancement, with respect to the nonrepumping case, is well reproduced by a simple rate equation model, which also describes single-laser repumping schemes reported previously. The repumping scheme is limited by a weak additional loss channel into the long-lived 5s5p3P05s5p{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{0}5s5p3P0​ state. For low repumping intensities, the signature of a halo formed by magnetically trapped atoms in the metastable state is found.
We explore the efficacy of two single-repumping schemes, 5 s 5 p P 2 3 − 5 p 2 P 2 3 ( 481 nm ) and 5 s 5 p P 2 3 − 5 s 5 d D 2 3 ( 497 nm ) , for a magneto-optical trap (MOT) of Sr atoms. We reveal that the enhancement in the MOT lifetime is limited to 26.9(2) for any single-repumping scheme. Our investigation indicates that the primary decay path from the 5 s 5 p P 1 1 state to the 5 s 5 p P 0 3 state proceeds via the 5 s 4 d D 1 3 state, rather than through the upper states accessed by the single-repumping lasers. We estimate that the branching ratio for the 5 s 5 p P 1 1 → 5 s 4 d D 1 3 → 5 s 5 p P 0 3 decay path is 1 : 3.9 ( 1.5 ) × 10 6 and the decay rate for the transition from the 5 s 5 p P 1 1 state to the 5 s 4 d
We present a faster repumping scheme for strontium magneto-optical traps operating on the broad 5 s 2 1 S 0 − 5 s 5 p 1 P 1 laser cooling transition. Contrary to existing repumping schemes, we directly address lost atoms that spontaneously decayed to the 5 s 4 d 1 D 2 state, sending them back into the laser cooling cycle by optical pumping on the 5 s 4 d 1 D 2 − 5 s 8 p 1 P 1 transition. We thus avoid the ∼ 100 µ s -slow decay path from 5 s 4 d 1 D 2 to the 5 s 5 p 3 P 1 , 2 states that is part of other repumping schemes. Using one low-cost external-cavity diode laser emitting at 448 nm , we show our scheme increases the flux out of a 2D magneto-optical trap by 60 % compared to without repumping. Furthermore, we perform spectroscopy on the 5 s 4 d 1 D 2 − 5 s 8 p 1 P 1 transition and measure its frequency ν Sr 88 = ( 668917515.3 ± 4.0 ± 25 ) MHz . We also measure the frequency shifts between the four stable isotopes of strontium and infer the specific mass and field shift factors, δ ν SMS 88 , 86 =<
We report on a novel mechanism that allows for strong laser cooling of atoms that do not have a closed cycling transition. This mechanism is observed in a magneto-optical trap (MOT) for erbium, an atom with a very complex energy level structure with multiple pathways for optical-pumping losses. We observe surprisingly high trap populations of over 10(6) atoms and densities of over 10(11) atoms cm(-3), despite the many potential loss channels. A model based on recycling of metastable and ground state atoms held in the quadrupole magnetic field of the trap explains the high trap population, and agrees well with time-dependent measurements of MOT fluorescence. The demonstration of trapping of a rare-earth atom such as erbium opens a wide range of new possibilities for practical applications and fundamental studies with cold atoms.
We present an experimental, numerical, and analytical study of strontium magneto-optical trap (MOT) loading from a cold atomic beam in a configuration optimized for high numerical aperture optical tweezers. Our approach orients the beam flow along the MOT symmetry axis to reduce the experimental complexity and maximize the overall optical access into the scientific region of study. We use a moving molasses technique to enable this configuration and show that its performance depends critically on metastable-state shelving (to 5s5p 3P2) during the atom transfer to the three-dimensional (3D) MOT. Furthermore, we find that the parameters for optimal transfer efficiency are bounded by dark-state loss (to 5s5p 3P0) in the trap region where repumping is present. These observations are verified to great degree of accuracy using both our developed analytical and numerical models. The corresponding 3D simulation tool is used to perform a comprehensive study of the trap loading dynamics, beginning at the oven exit and ending at the 3D MOT, demonstrating its effectiveness in optimizing an effusive oven experiment.