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arXiv:1202.2165 · Journal of Physics B Atomic Molecular and Optical Physics

Inter-isotope effects in optimal dual-isotope loading into a shallow optical trap

M S Hamilton, A R Gorges, J L Roberts

Published Apr 19, 2012• 7 citations
arXivHTMLPDF

Abstract

Examination of loading the isotopes $^{85}$Rb and $^{87}$Rb simultaneously into a shallow far-off-resonance trap (FORT) has revealed an unexpected decrease in maximum atom number loaded as compared to loading either isotope alone. The simultaneous loading of the FORT will be affected by additional homonuclear and heteronuclear light-assisted collisional losses. However, these losses are measured and found to be insufficient to explain the observed drop in total number of atoms loaded into the FORT. We find that our observations are consistent with a decrease in loading rate caused by inter-isotope disruptions of the efficient laser cooling required to load atoms into the optical trap.

DOI
10.1088/0953-4075/45/9/095302

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Examination of loading the isotopes 85^{85}85Rb and 87^{87}87Rb simultaneously into a shallow far-off-resonance trap (FORT) has revealed an unexpected decrease in maximum atom number loaded as compared to loading either isotope alone. The simultaneous loading of the FORT will be affected by additional homonuclear and heteronuclear light-assisted collisional losses. However, these losses are measured and found to be insufficient to explain the observed drop in total number of atoms loaded into the FORT. We find that our observations are consistent with a decrease in loading rate caused by inter-isotope disruptions of the efficient laser cooling required to load atoms into the optical trap.
We have studied heteronuclear and homonuclear excited-state--ground-state collisions by loading both 85Rb^{85}\mathrm{Rb}85Rb and 87Rb^{87}\mathrm{Rb}87Rb into a far off resonant trap (FORT). Because of the relatively weak confinement of the FORT, we expect the hyperfine structure of the different isotopes to play a crucial role in the collision rates. This dependence on hyperfine structure allows us to measure collisions associated with long-range interatomic potentials of different structures, such as long and short ranged; or such as purely attractive, purely repulsive, or mixed attractive and repulsive. We observe significantly different loss rates for different excited-state potentials. Additionally, we observe that some collisional channels' loss rates are saturated at our operating intensities (15mW∕cm2)(15\phantom{\rule{0.3em}{0ex}}\mathrm{mW}∕{\mathrm{cm}}^{2})(15mW∕cm2). These losses are important limitations in loading dual isotope optical traps.
We have studied the effects of loading 87^{87}87Rb into a far off resonant trap (FORT) in the presence of an ultracold cloud of 85^{85}85Rb. The presence of the 85^{85}85Rb resulted in a marked decrease of the 87^{87}87Rb load rate. This decrease is consistent with a decrease in the laser cooling efficiency needed for effective loading. While many dynamics which disrupt loading efficency arise when cooling in a dense cloud of atoms (reabsorption, adverse optical pumping, etc.), the large detuning between the transitions of 85^{85}85Rb and 87^{87}87Rb should isolate the isotopes from these effects. For our optical molasses conditions we calculate that our cooling efficiencies require induced ground-state coherences. We present data and estimates which are consistent with heteronuclear long-ranged induced dipole-dipole collisions disrupting these ground state coherences, leading to a loss of optical trap loading efficiency.
We have examined loading of 85Rb^{85}\mathrm{Rb}85Rb atoms into a shallow far-off-resonance trap (FORT) from an optical molasses stage following a magneto-optical trap (MOT) stage and compared it to loading from a MOT stage only. Substantially more atoms could be loaded into the FORT using an optical molasses. To determine why this was the case, we measured the rate of atoms loaded into the FORT and the losses from the FORT during both loading processes over a range of detunings and hyperfine pump powers. We found that the losses induced during MOT loading were essentially the same as the losses induced during molasses loading and decreased with increased red detuning; however, we found that the differences in load rate caused an optical molasses to optimize at a greater detuning, improving the number of atoms we could trap by a factor of 2 over that of optimal loading from a MOT.
Summary form only given. We have studied collisional effects in the presence of light by investigating the loading times of the trap under different conditions. We have observed shorter loading times for the Na trap in the presence of either Rb isotope, compared with a single Na trap for different intensities of 589-nm light. In contrast, the loading times for the Rb isotopes were not measurably affected by the presence of Na (a similar effect was observed in an Na-Cs trap, in which the Cs trap was unaffected).
Abstract We report simultaneous cooling and trapping of metastable 84 Kr ( 84 Kr*) and metastable 86 Kr ( 86 Kr*) atoms by overlapping the cooling laser beams for these different isotopes in the same region of a magneto-optical trap (MOT). Approximately 2 × 10 5 atoms of each isotope were trapped in this two isotope-MOT (TIMOT). We have investigated the heteronuclear collision trap loss rates for each isotope due to the presence of cold atoms of other isotope using the TIMOT loading curves. The two body heteronuclear loss rate coefficient (i.e. for the loss of 84 Kr* due to presence of 86 Kr*) and the reciprocal loss rate coefficient (i.e. for the loss of 86 Kr* due to presence of 84 Kr*) are measured to be (8.7 ± 0.8) × 10 −10 cm 3 s −1 and (8.8 ± 0.8) × 10 −10 cm 3 s −1 respectively for laser beam intensity values of 21 mW cm −2 (for cooling of 84 Kr*) and 64 mW cm −2 (for cooling of 86 Kr*). The dependence of heteronuclear cold collision loss rate on cooling laser beams intensity has also been studied and compared with homonuclear cold collision loss rate.
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.