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Physical Review Letters

Bimodal speed distributions in laser-cooled atoms

Yaakov Shevy, David S. Weiss, P. J. Ungar, Steven Chu

Published Mar 6, 1989• 45 citations
Publisher

Abstract

We report a bimodal speed distribution for sodium atoms cooled in ``optical molasses.'' The temperatures of the two velocity groups differ in their dependence on laser detuning, sensitivity to beam misalignment and imbalance, polarization, and the presence of static magnetic fields. We believe that this behavior is evidence for two distinct cooling mechanisms.

DOI
10.1103/physrevlett.62.1118

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The general features of optical molasses,1 continuously loaded with slow sodium atoms, are described. Three orthogonal pairs of counterpropagating laser beams, tuned slightly below resonance, capture and viscously confine the slow atoms which are produced continuously by a Zeeman tuned laser-cooled atomic beam.2 The continuous loading of this molasses region is further enhanced by using optical depumping techniques (optically pumping the atoms out of the cooling cycle with another laser) to extract more slow atoms from the beam than are otherwise available. Results from simple analytic theories and Monte Carlo simulations of the molasses are compared to the experimental findings. In particular, we look at the molasses lifetime as a function of beam imbalance and laser frequency. Although some features of the molasses can be explained (in particular at low intensity) many discrepancies are noted. One most obvious of these is the relative insensitivity of the lifetime to intensity imbalance, the discrepancy being at least an order of magnitude in added rate.

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The transverse velocity distribution of a sodium atomic beam, laser cooled in one dimension, has been measured with sub-recoil resolution. A velocity-selective detection system was employed to separate the longitudinal and transverse velocity components to provide a direct measure of the transverse velocity distribution. The effects of varying the cooling laser intensity and detuning on the derived transverse temperature are presented, and show a remarkable insensitivity to the laser intensity. Atomic velocities within a factor of two of the recoil velocity are

Quantum and Semiclassical Optics Journal of the European Optical Society Part B•Jun 1, 1996•7 citations
We report a bimodal speed distribution for sodium atoms cooled in ``optical molasses.'' The temperatures of the two velocity groups differ in their dependence on laser detuning, sensitivity to beam misalignment and imbalance, polarization, and the presence of static magnetic fields. We believe that this behavior is evidence for two distinct cooling mechanisms.
Since the first optical molasses1 came out, the temperature of the atomic vapor in the optical molasses, which was determined by measuring the time of flight (TOF), was found to be well below the Doppler cooling limit,2 which is based on the assumption that a single frequency laser interacts with the two-energy-level atoms. The new theory3 proposes that this lower temperature is due to the nonadiabatic motion of the atoms with Zeeman sublevels in the optical field with polarization gradients. A bimodal speed distribution in optical molasses was also observed.4
Xenon atoms are released from a magneto-optical trap and equilibrate in a one-dimensional, ${\mathrm{\ensuremath{\sigma}}}_{+}$-${\mathrm{\ensuremath{\sigma}}}_{\mathrm{\ensuremath{-}}}$ optical molasses, along which a uniform magnetic field is applied. In the rest frame of the ``corkscrew'' pattern of laser polarization, the magnetic field induces a nonzero average atomic velocity. Introducing a moving molasses through a relative difference in the two laser-beam frequencies allows straightforward, accurate measurements of the molasses-frame velocity distribution over a large range of magnetic fields. The measurements reveal the loss of polarization-gradient cooling in fields higher than \ensuremath{\sim}0.2 mT, in quantitative agreement with theoretical predictions. \textcopyright{} 1996 The American Physical Society.
Optical molasses is a three-dimensional (3-D) configuration of laser beams used to laser-cool and to viscously confine neutral atoms. Atoms laser cooled in optical molasses reach temperatures much lower than the limit given by the original theories of laser cooling based on the Doppler effect. This cooling below the Doppler-cooling limit is now seen as being due to new laser cooling mechanisms not considered in the original theories. The dependence of the atomic temperature on parameters such as laser intensity and detuning shows good agreement between calculations performed in 1-D and experiments performed in 3-D. For cooling of Na and Cs atoms, the lowest observed temperatures correspond to rms velocities between three and four times the single photon recoil velocity. For Cs the temperature is 2.5 ± 0.6 μK and is the lowest temperature ever measured for 3-D cooling.
The general features of optical molasses,1 continuously loaded with slow sodium atoms, are described. Three orthogonal pairs of counterpropagating laser beams, tuned slightly below resonance, capture and viscously confine the slow atoms which are produced continuously by a Zeeman tuned laser-cooled atomic beam.2 The continuous loading of this molasses region is further enhanced by using optical depumping techniques (optically pumping the atoms out of the cooling cycle with another laser) to extract more slow atoms from the beam than are otherwise available. Results from simple analytic theories and Monte Carlo simulations of the molasses are compared to the experimental findings. In particular, we look at the molasses lifetime as a function of beam imbalance and laser frequency. Although some features of the molasses can be explained (in particular at low intensity) many discrepancies are noted. One most obvious of these is the relative insensitivity of the lifetime to intensity imbalance, the discrepancy being at least an order of magnitude in added rate.
The transverse velocity distribution of a sodium atomic beam, laser cooled in one dimension, has been measured with sub-recoil resolution. A velocity-selective detection system was employed to separate the longitudinal and transverse velocity components to provide a direct measure of the transverse velocity distribution. The effects of varying the cooling laser intensity and detuning on the derived transverse temperature are presented, and show a remarkable insensitivity to the laser intensity. Atomic velocities within a factor of two of the recoil velocity are