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Journal of the Physical Society of Japan

Microwave-induced Evaporation in a Crossed Dipole Trap

D. J. Han

Published Feb 13, 2007• 0 citations
Publisher

Abstract

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.

DOI
10.1143/jpsj.76.023301

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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 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.
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/cm3{\mathrm{m}}^{3}m3 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/cm3{\mathrm{m}}^{3}m3. This corresponds to a factor of 28 increase in atomic phase-space density.
Most cold atoms experiments in microgravity platforms or in Space are achieved using atom chips, leading to limitations in terms of optical access and inhomogeneous magnetic fields. Optical dipole traps do not have these drawbacks but have difficulties producing atomic samples with a large number of atoms at ultra low temperature in the absence of gravity. Here, we report on an efficient evaporative cooling in two-crossed laser beams during parabolic flights. Time-averaged potentials combine the advantages of large capture volume and trap compression, increasing the initial phase space density and collision rate to favor the evaporative process. With this technique we demonstrate the production of an ultra cold gas of 2.5×1042.5\times 10^42.5×104 rubidium atoms at a temperature below 100 nK in less than 4 seconds. Our experiment paves the way for the development of quantum sensors applied to fundamental physics and geodesy as well as the study of ultracold atomic physics in Space.
Abstract Most cold atoms experiments in microgravity platforms or in space are achieved using atom chips, constrained by limited optical access and inhomogeneous magnetic fields. Despite their higher power consumption, optical dipole traps are interesting alternatives because they do not have these drawbacks. Nevertherless, producing samples with a large number of ultracold atoms in a dipole trap, in the absence of gravity, has remained challenging so far. Here, we report on an efficient evaporative cooling in two-crossed laser beams during parabolic flights. Time-averaged potentials combine the advantages of large capture volume and trap compression, increasing the initial phase space density and collision rate to favor the evaporative process. With this technique we demonstrate the production of an ultra cold gas of 2.3 × 10 4 rubidium atoms at a temperature below 100 nK in less than 4 seconds. Our experiment paves the way for the development of quantum sensors applied to fundamental physics and geodesy as well as the study of ultracold atomic physics in space.
We propose and study a model of forced evaporation of atomic clouds in crossed-beam optical dipole traps that explicitly includes the growth of a population in the ``wings'' of the trap and its subsequent impact on dimple temperature and density. It has long been surmised that a large wing population is an impediment to the efficient production of Bose-Einstein condensates in crossed-beam traps. Understanding the effect of the wings is particularly important for $\ensuremath{\lambda}=1.06\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ traps, for which a large ratio of Rayleigh range to beam waist results in wings that are large in volume and extend far from the dimple. Key ingredients to our model's realism are (1) our explicit treatment of the nonthermal, time-dependent energy distribution of wing atoms in the full anharmonic potential and (2) our accurate estimations of transition rates among dimple, wing, and free-atom populations, obtained with Monte Carlo simulations of atomic trajectories. We apply our model to trap configurations in which neither, one, or both of the wing potentials are made unbound by applying a ``tipping'' gradient. We find that forced evaporation in a trap with two bound wing potentials produces a large wing population which can collisionally heat the dimple so strongly as to preclude reaching quantum degeneracy. Evaporation in a trap with one unbound wing, such as that made by crossing one vertical beam and one horizontal beam, also leads to a persistent wing population which dramatically degrades the evaporation process. However, a trap with both wings tilted so as to be just unbound enjoys a nearly complete recovery of efficient evaporation. By introducing to our physical model an ad hoc, tunable escape channel for wing atoms, we study the effect of partially filled wings, finding that a wing population caused by single-beam potentials can drastically slow down evaporative cooling and increase the sensitivity to the choice of $\ensuremath{\eta}$.
We demonstrate experimentally the evaporative cooling of a few hundred rubidium-87 atoms in a single-beam microscopic dipole trap. Starting with 800 atoms at a temperature of 125 $\ensuremath{\mu}$K, we produce an unpolarized sample of 40 atoms at 110 nK, within 3 s. The phase-space density at the end of the evaporation reaches unity, close to quantum degeneracy. The gain in phase-space density after evaporation is 103{10}^{3}103. We find that the scaling laws used for much larger numbers of atoms are still valid despite the small number of atoms involved in the evaporative cooling process. We also compare our results to a simple kinetic model describing the evaporation process and find good agreement with the data.