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Journal of Physics B Atomic Molecular and Optical Physics

Loading a39K crossed optical dipole trap from a magneto-optical trap

Bruno Marangoni, Carlos Renato Menegatti, Luís Gustavo Marcassa

Published Aug 9, 2012• 7 citations
Publisher

Abstract

In this work, we have applied sub-Doppler laser cooling to a 39 K magneto-optical trap in order to load a 1071 nm crossed optical dipole trap. The number of atoms loaded into the dipole trap was characterized as a function of the frequency and intensity of the cooling and repump laser beams. For the optimum conditions, the dipole trap has about 2 × 10 6 atoms at an atomic density of 2 × 10 12 cm −3 , with a temperature of about 10 µK. This technique is a very simple procedure to load a 39 K optical dipole trap without a previous magnetic evaporative cooling step and may find application in other atomic physic systems.

DOI
10.1088/0953-4075/45/17/175301

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Simultaneous loading of 39 K and Rb into a crossed dipole trap: Characterization and two-body losses

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In this work, a broadband 1071 nm crossed optical dipole trap was loaded simultaneously from a two-atomic-species K-Rb magneto-optical trap. The number of atoms loaded into the dipole trap, for both species, was characterized as a function of different laser parameters and loading procedures. The best observed loading procedure for K is explained by the ac Stark shift of the hyperfine states involved in the ${D}_{2}$ transition. For the optimum conditions, the dipole trap has about $3\ifmmode\times\else\texttimes\fi{}{10}^{6}$ ${}^{39}$K atoms at an atomic density of $7\ifmmode\times\else\texttimes\fi{}{10}^{12}$ cm${}^{\ensuremath{-}3}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ Rb atoms at an atomic density of $5\ifmmode\times\else\texttimes\fi{}{10}^{12}$ cm${}^{\ensuremath{-}3}$, with temperatures of 22 and 10 $\ensuremath{\mu}$K, respectively. Finally, by studying the time evolution of both species trapped in the crossed optical dipole trap, we have observed a nonexponential decay associated with heteronuclear two-body losses. Such losses may be due to KRb photoassociation by the broadband 1071 nm laser.

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•Jan 1, 2014•48 citations
In this work, we have applied sub-Doppler laser cooling to a 39 K magneto-optical trap in order to load a 1071 nm crossed optical dipole trap. The number of atoms loaded into the dipole trap was characterized as a function of the frequency and intensity of the cooling and repump laser beams. For the optimum conditions, the dipole trap has about 2 × 10 6 atoms at an atomic density of 2 × 10 12 cm −3 , with a temperature of about 10 µK. This technique is a very simple procedure to load a 39 K optical dipole trap without a previous magnetic evaporative cooling step and may find application in other atomic physic systems.
In this work, a broadband 1071 nm crossed optical dipole trap was loaded simultaneously from a two-atomic-species K-Rb magneto-optical trap. The number of atoms loaded into the dipole trap, for both species, was characterized as a function of different laser parameters and loading procedures. The best observed loading procedure for K is explained by the ac Stark shift of the hyperfine states involved in the D2{D}_{2}D2​ transition. For the optimum conditions, the dipole trap has about $3\ifmmode\times\else\texttimes\fi{}{10}^{6}$ 39{}^{39}39K atoms at an atomic density of $7\ifmmode\times\else\texttimes\fi{}{10}^{12}$ cm${}^{\ensuremath{-}3}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ Rb atoms at an atomic density of $5\ifmmode\times\else\texttimes\fi{}{10}^{12}$ cm${}^{\ensuremath{-}3}$, with temperatures of 22 and 10 $\ensuremath{\mu}$K, respectively. Finally, by studying the time evolution of both species trapped in the crossed optical dipole trap, we have observed a nonexponential decay associated with heteronuclear two-body losses. Such losses may be due to KRb photoassociation by the broadband 1071 nm laser.
We discuss our experimental results on forced evaporative cooling of cold rubidium 87 Rb atoms to quantum degeneracy in an Optical Dipole Trap. The atoms are first trapped and cooled in a magneto-optical trap (MOT) loaded from a continuous beam of cold atoms [1]. More than 10 10 atoms are trapped in the MOT and then about 10 8 atoms are transferred to a Quasi-Electrostatic Trap (QUEST) formed by tightly focused CO2 laser (λ = 10.6μ m ) beams intersecting at their foci in an orthogonal configuration in the horizontal plane. Before loading the atoms into the dipole trap, the phase-space density of the atomic ensemble was increased making use of sub-doppler cooling at large detuning and the temporal dark MOT technique. In a MOT the phase-space density of the atomic ensemble is six orders of magnitude less than what is required to achieve quantum degeneracy. After transferring atoms into the dipole trap efficiently, phase-space density increases by a factor of 10 3 . Further increase in phase-space density to quantum degeneracy is achieved by forced evaporative cooling of atoms in the dipole trap. The evaporative cooling process involves a gradual reduction of the trap depth by ramping down the trapping laser intensity over a second. The temperature of the cold atomic cloud was measured by time-of-flight (TOF) technique. The spatial distribution of the atoms is measured using absorption imaging. We report results of evaporative cooling in a single beam and in a crossed double-beam dipole traps. Due to the large initial phase space density, and large initial number of atoms trapped, the quantum phase transition occurs after about 600 ms of evaporative cooling in our optimized crossed dipole trap.
We present our technique to create a magneto-optical trap (MOT) for dysprosium atoms using the narrow-line cooling transition at 626 nm to achieve suitable conditions for direct loading into an optical dipole trap. The MOT is loaded from an atomic beam via a Zeeman slower using the strongest atomic transition at 421 nm. With this combination of two cooling transitions we can trap up to 2.0·10(8) atoms at temperatures down to 6 μK. This cooling approach is simpler than present work with ultracold dysprosium and provides similar starting conditions for a transfer to an optical dipole trap.
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.
The main goal of this research is to cool potassium-41 atoms to ultracold temperatures (<10 μK) in order to study the spin dynamics of atom to atom interactions. However, our current goal is to finish constructing the apparatus that will allow us to confine and cool these atoms in three dimensions. This apparatus will cool and confine the potassium atoms in 3 steps. First, the atoms will be cooled and trapped in our 3D magneto-optical trap. This step of the experiment will cool the atoms near the Doppler-cooling limit, which for potassium atoms is 145 μK. Second, atoms will be loaded into a FORT (far off resonance trap) with an expected depth of 25 μK. In order to cool the atoms below the Doppler-cooling limit, we will use sub-Doppler cooling techniques. What is particularly unique about our experiment is that we plan to use an octupole magnetic field to achieve sub-Doppler cooling. In typical laser cooling experiments sub-Doppler temperatures are achieved when the quadrupole magnetic field is off, which leaves the atoms untrapped. Third, once the atoms have been loaded into the FORT one last cooling mechanism will be implemented. This mechanism is referred to as evaporative cooling and will cool the atoms below 10 μK.
We present our technique to create a magneto-optical trap for dysprosium atoms using the narrow-line cooling transition at 626 \,nm to achieve suitable conditions for direct loading into an optical dipole trap. The magneto-optical trap is loaded from an atomic beam via a Zeeman slower using the strongest atomic transition at 421 \,nm. With this combination of two cooling transitions we can trap up to 2.0⋅1082.0\cdot10^82.0⋅108 atoms at temperatures down to 6 μ\, \muμK. This cooling approach is simpler than present work with ultracold dysprosium and provides similar starting conditions for a transfer to an optical dipole trap.