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Physical Review A

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

Carlos Renato Menegatti, Bruno Marangoni, Jonathan Tallant, Luís Gustavo Marcassa

Published Aug 9, 2013• 4 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.88.023411

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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 D2{D}_{2}D2​ transition. For the optimum conditions, the dipole trap has about 3\ifmmode×\else\texttimes\fi1063\ifmmode\times\else\texttimes\fi{}{10}^{6}3\ifmmode×\else\texttimes\fi106 39{}^{39}39K atoms at an atomic density of 7\ifmmode×\else\texttimes\fi10127\ifmmode\times\else\texttimes\fi{}{10}^{12}7\ifmmode×\else\texttimes\fi1012 cm\ensuremath−3{}^{\ensuremath{-}3}\ensuremath−3 and 2\ifmmode×\else\texttimes\fi1062\ifmmode\times\else\texttimes\fi{}{10}^{6}2\ifmmode×\else\texttimes\fi106 Rb atoms at an atomic density of 5\ifmmode×\else\texttimes\fi10125\ifmmode\times\else\texttimes\fi{}{10}^{12}5\ifmmode×\else\texttimes\fi1012 cm\ensuremath−3{}^{\ensuremath{-}3}\ensuremath−3, with temperatures of 22 and 10 \ensuremathμ\ensuremath{\mu}\ensuremathμK, 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.
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.
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.
We realize a combined trap for bosonic chromium (52Cr) and rubidium (87Rb) atoms. Initial experiments focus on exploring a suitable loading scheme for the combined trap and on studies of new trap loss mechanisms originating from simultaneous trapping of two species. By comparing the trap loss from the 87Rb magneto-optical trap (MOT) in the absence and presence of magnetically trapped ground state 52Cr atoms we determine the scattering cross-section of σ inel,RbCr = 5.0 ± 4.0 × 10−18 m2 for light-induced inelastic collisions between the two species. Studying the trap loss from the Rb magneto-optical trap induced by the Cr cooling-laser light, the photoionization cross-section of the excited 5P3/1 state at an ionizing wavelength of 426 nm is measured to be σ P = 1.1 ± 0.3 × 10−21 m2.
The first simultaneous cooling and confinement of two different atomic species from opposite sides of the periodic table in a dual magneto optical trap (DMOT) has been accomplished. The alkali-metal 85Rb and the noble gas 40Ar* have been simultaneously confined, characterized, and interspecies interaction parameters have been measured. The DMOT confined 1.2 × 106 85Rb atoms at a density of 1 × 1010/cm3 and 1.4 × 106 40Ar* atoms with a density of 1.2 × 1010/cm3. A collisional loss rate coefficient for Rb-Ar* has been determined to be γRb-Ar, = (4.8 ± 1.6) × 10−11cm3/s. A typical reduction of 3% of the florescence of the trapped 85Rb was observed when the 40Ar* trap was present. The loss of atoms from the dual trap is presumed to be caused by radiative escape and ionization losses, for an 40Ar* MOT it is due to Penning and associative ionization losses. An Ar* PI/AI ionization ratio of 6.7 ± 3.6 was determined, which makes the Penning ionization rate the dominant loss mechanism. The ionization rate was obtained with a SRS RGA200 mass spectrum analyzer specifically modified to work in conjunction with an ion optical field plate setup to collect the ions expelled from the MOT. The 85Rb was trapped with a 780 nm diode laser while the 40Ar* was slowed and trapped with a 811 nm Ti:sapphire laser, which was specifically modified to be locked to within 1 MHz of the atomic transition with a linewidth of ∼1 MHz. The metastable state 40Ar* was produced with a rf driven resonating cavity discharge which produced an atomic beam with an angular flux density of 4 × 1014 atoms s−1 sr−1.
We present an optimized strategy for the production of tightly confined Bose-Einstein condensates (BEC) of 87{}^{87}87Rb in a crossed dipole trap with direct loading from a magneto-optical trap. The dipole trap is created with light of a multifrequency fiber laser with a center wavelength of 1070 nm. Evaporative cooling is performed by ramping down the laser power only. A comparison of the resulting atom number in an almost pure BEC to the initial atom number and the value for the gain in phase space density per atom lost confirm that this straightforward strategy is very efficient. We observe that the temporal characteristics of evaporation sequence are strongly influenced by power-dependent two-body losses resulting from enhanced optical pumping to the higher-energy hyperfine state. We characterize these losses and compare them to results obtained with a single-frequency laser at 1030 nm.
We demonstrate the simultaneous magneto-optical trapping (MOT) of Rb and Sr and examine the characteristic loss of Rb in the MOT due to photoionization by the cooling laser for Sr. The photoionization cross section of Rb in the 5P3/25{P}_{3/2}5P3/2​ state at 461 nm is determined to be $1.4(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$ cm2{}^{2}2. It is important to consider this loss rate to realize a sufficiently large number of trapped Rb atoms to achieve a quantum degenerate mixture of Rb and Sr.