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Physical Review A

Trap loss in a rubidium crossed dipole trap by short-range photoassociation

Carlos Renato Menegatti, Bruno Marangoni, Nadia Bouloufa-Maafa, Olivier Dulieu, Luís Gustavo Marcassa

Published May 8, 2013• 24 citations
Publisher

Abstract

Significant two-body losses are observed in an ultracold gas of ${}^{85}$Rb or ${}^{87}$Rb atoms in a crossed dipole trap implemented with a broadband laser with a wavelength around 1071 nm. Using available spectroscopic data on the excited states of Rb${}_{2}$, as well as accurate computed transition dipole moment functions, we interpret these losses as due to photoassociation of deeply bound levels of the 0${}_{u}^{+}$ coupled states by the trapping laser, followed by spontaneous emission down to bound levels of the Rb${}_{2}$ ground state thus forming ultracold molecules. The observed ratio of 3.3 between the loss rates of ${}^{87}$Rb and ${}^{85}$Rb is well reproduced by the theoretical model.

DOI
10.1103/physreva.87.053404

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arXiv:1211.6533•New Journal of Physics•Apr 19, 2013•3 citations
Significant two-body losses are observed in an ultracold gas of 85{}^{85}85Rb or 87{}^{87}87Rb atoms in a crossed dipole trap implemented with a broadband laser with a wavelength around 1071 nm. Using available spectroscopic data on the excited states of Rb2{}_{2}2​, as well as accurate computed transition dipole moment functions, we interpret these losses as due to photoassociation of deeply bound levels of the 0u+{}_{u}^{+}u+​ coupled states by the trapping laser, followed by spontaneous emission down to bound levels of the Rb2{}_{2}2​ ground state thus forming ultracold molecules. The observed ratio of 3.3 between the loss rates of 87{}^{87}87Rb and 85{}^{85}85Rb is well reproduced by the theoretical model.
Abstract The influence of a high-power single-frequency fiber laser on the formation of ultracold 85 Rb 2 molecules is investigated as a function of its frequency (in the 1062–1070 nm range) in a magneto-optical trap. We find evidence for the formation of ground-state 85 Rb 2 molecules in low vibrational levels ( v ≤ 20 ) with a maximal rate of 10 4 s −1 , induced by short-range photoassociation by the fiber laser followed by spontaneous emission. When this laser is used to set up a dipole trap, we measure an atomic loss rate at a wavelength far from the PA resonances, only four times smaller than that observed at a PA resonance wavelength. This work may have important consequences for atom trapping using lasers around the conventional 1064 nm wavelength.
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.
The influence of a high-power single-frequency fiber laser on the formation of ultracold 85Rb2 molecules is investigated as a function of its frequency (in the 1062–1070 nm range) in a magneto-optical trap. We find evidence for the formation of ground-state 85Rb2 molecules in low vibrational levels () with a maximal rate of 104 s−1, induced by short-range photoassociation by the fiber laser followed by spontaneous emission. When this laser is used to set up a dipole trap, we measure an atomic loss rate at a wavelength far from the PA resonances, only four times smaller than that observed at a PA resonance wavelength. This work may have important consequences for atom trapping using lasers around the conventional 1064 nm wavelength.
Measurements of the temperature dependence of trap loss collisions of laser-cooled rubidium atoms are presented. A significant suppression of the rate of collisions between ground- and excited-state atoms is observed for temperatures below $\\sim$70 mu\\mumuK. In such inelastic collisions, the atomic excitation takes place at long range and must survive against spontaneous decay to short range. Our results are consistent with recent predictions for the low temperature suppression of these collisions due to reduced survival of atomic excitation at low collision velocities.^ We report on measurements of trap loss collisions between ultracold Rb atoms which are confined in a magneto-optical trap. Both isotopes sp85\\sp{85}sp85Rb and sp87\\sp{87}sp87Rb have been studied over wide range of trap laser intensities and detunings. The trap loss collision rates exhibit variations over 3 orders of magnitude. We observe significant differences between the isotopes. At large detunings the ability of the trap to recapture products of the hyperfine-changing collisions is significantly diminished. This finding is supported by our numerical simulations of the recapture process. The trap loss rates due to collisions with room temperature background gas have also been measured.^ Using laser light tuned to a repulsive molecular potential, we have been able to suppress inelastic ground-state hyperfine changing collisions between ultracold sp87\\sp{87}sp87Rb atoms. Adiabatic excitation to the repulsive curve alters the atomic trajectories and prevents the atoms from approaching close enough for the hyperfine change to occur. Experimental results show suppressions up to $\\sim$50% and are in reasonable agreement with a simple Landau-Zener model.^ Finally, we have observed a significant cooperative effect in ultracold trap loss collisions induced by two separate lasers. One laser, tuned close to the atomic resonance, excites the atom pair to an attractive potential at long range. The resulting acceleration and deflection give rise to an enhancement in the collisional flux of ground-state atoms reaching short range, as probed by a second laser. Enhancements up to a factor of $\\sim$3 have been observed. ^
We have confined up to 10{sup 4} {sup 85}Rb atoms in an optical dipole force trap operating up to 65 nm from atomic resonance. For detunings greater than 16 nm, we obtained long confinement times, limited only by background gas collisions, without additional cooling. However, for trapping laser tunings between 4 nm and 16 nm to the red of the lowest Rb (D1) resonance, we observed a novel trap loss mechanism which results in much shorter-trap lifetimes. The trap loss rate is very sensitive to the laser wavelength, and exhibits a complex spectrum. This trap loss may arise from a photo-associative collisional process in which an initially free pair of colliding ground state atoms is promoted to a bound molecular excited state. The bound excited molecule could then be lost from the trap by photoionization, or by spontaneous decay to an untrapped ground state. The results of our preliminary experiments and of further experiments to clarify this novel trap loss process will be presented.
Utilizing single-photon photoassociation, we have achieved ultracold rubidium molecules with a high number density that provides a new efficient approach toward molecular quantum degeneracy. A new detection mechanism for ultracold molecules utilizing inelastic atom–molecule collision is demonstrated. The resonant coupling effect on the formation of the X 1 Σ + g ground state 85 Rb 2 allows for a sufficient number of more deeply bound ultracold molecules, which induced an additional trap loss and heating of the co-existing atoms owing to the inelastic atom–molecule collision. Therefore, after the photoassociation process, the ultracold molecules can be investigated using the absorption image of the ultracold rubidium atoms mixed with the molecules in a crossed optical dipole trap. The existence of the ultracold molecules was then verified, and the amount of accumulated molecules was measured. This method detects the final produced ultracold molecules, and hence is distinct from the conventional trap loss experiment, which is used to study the association resonance. It is composed of measurements of the time evolution of an atomic cloud and a decay model, by which the number density of the ultracold 85 Rb 2 molecules in the optical trap was estimated to be >5.2 × 10 11 cm −3 .