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Physical Review Letters

Evaporative Cooling in a Crossed Dipole Trap

Charles S. Adams, Heun Jin Lee, Nir Davidson, Mark A. Kasevich, Steven Chu

Published May 1, 1995• 211 citations
Publisher

Abstract

Laser cooled sodium atoms are trapped in an optical dipole force trap formed by the intersection of two 1.06 \ensuremath{\mu}m laser beams. Densities as high as $4\ifmmode\times\else\texttimes\fi{}{10}^{12}$ atoms/c${\mathrm{m}}^{3}$ at a temperature of $\ensuremath{\sim}140$ \ensuremath{\mu}K have been obtained in a $\ensuremath{\sim}900$ \ensuremath{\mu}K deep trap. By reducing the trap depth over a 2 s interval, we have evaporatively cooled the atoms to a final temperature of $\ensuremath{\sim}4$ \ensuremath{\mu}K at a density of $6\ifmmode\times\else\texttimes\fi{}{10}^{11}$ atoms/c${\mathrm{m}}^{3}$. This corresponds to a factor of 28 increase in atomic phase-space density.

DOI
10.1103/physrevlett.74.3577

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Quantum Electronics and Laser Science Conference•Jun 25, 2003•0 citations
Laser cooled sodium atoms are trapped in an optical dipole force trap formed by the intersection of two 1.06 \ensuremath{\mu}m laser beams. Densities as high as 4\ifmmode×\else\texttimes\fi10124\ifmmode\times\else\texttimes\fi{}{10}^{12}4\ifmmode×\else\texttimes\fi1012 atoms/cm3{\mathrm{m}}^{3}m3 at a temperature of \ensuremath∼140\ensuremath{\sim}140\ensuremath∼140 \ensuremath{\mu}K have been obtained in a \ensuremath∼900\ensuremath{\sim}900\ensuremath∼900 \ensuremath{\mu}K deep trap. By reducing the trap depth over a 2 s interval, we have evaporatively cooled the atoms to a final temperature of \ensuremath∼4\ensuremath{\sim}4\ensuremath∼4 \ensuremath{\mu}K at a density of 6\ifmmode×\else\texttimes\fi10116\ifmmode\times\else\texttimes\fi{}{10}^{11}6\ifmmode×\else\texttimes\fi1011 atoms/cm3{\mathrm{m}}^{3}m3. This corresponds to a factor of 28 increase in atomic phase-space density.
We have Raman cooled sodium atoms below the photon recoil temperature in a novel type of blue-detuned optical dipole force trap. In this trap $4.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms have been cooled to an effective three dimensional temperature of 1.0 \ensuremath{\mu}K at a final density of $4\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{cm}}^{\ensuremath{-}3}$. No atoms were lost during the cooling process. The phase space density increased by a factor of 320 over the uncooled sample. This is the highest phase space density achieved by an all-optical cooling method.
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 a way of evaporative cooling using microwave-induced transitions in a crossed-beam optical dipole trap. By combining the optical and magnetic forces with gravity it allows to spatially select the most energetic atoms and drive them out of the trap by microwave. After 480 ms of evaporation, our data show the phase space density of the Cs atoms increases by a factor of 3.2 and the temperature is reduced from 2.4 µK to 400 nK. The cooling efficiency is mainly limited by the trap heating. This method provides another way to cool neutral atoms in optical trap and reach temperatures in the nano-Kevin regime.
We have observed evaporative cooling of magnetically trapped sodium atoms. A novel technique, rf induced evaporation, was used to reduce the temperature by a factor of 12 and increase the phase space density by more than 2 orders of magnitude. The elastic collision cross section of cold sodium atoms in the F=1F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1F=1, ${m}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}1$ hyperfine state was determined to be $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ cm2{\mathrm{m}}^{2}m2, which implies a positive value of the scattering length.
We have created a Bose-Einstein condensate (BEC) of 87Rb{}^{87}\mathrm{Rb}87Rb atoms directly in an optical trap. We employ a quasielectrostatic dipole force trap formed by two crossed CO2{\mathrm{CO}}_{2}CO2​ laser beams. Loading directly from a sub-Doppler laser-cooled cloud of atoms results in initial phase space densities of $\ensuremath{\sim}1/200$. Evaporatively cooling through the BEC transition is achieved by lowering the power in the trapping beams over $\ensuremath{\sim}2\mathrm{s}$. The resulting condensates are F=1F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1F=1 spinors with $3.5\ifmmode\times\else\texttimes\fi{}{10}^{4}$ atoms distributed between the ${m}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(\ensuremath{-}1,0,1)$ states.
Summary form only given. Recently, we have created a Bose condensate of /sup 87/Rb atoms directly in an optical trap formed by tightly focused laser beams. Following initial loading from a laser cooled gas, evaporative cooling through the BEC transition is achieved by simply lowering the depth of the optical trap. Our success is due in part to a high initial phase space density realized in the loading of our optical dipole trap and in part to the tight confinement of the atoms that permits rapid evaporative cooling to the BEC transition in 2 s.