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Annual Meeting Optical Society of America

Characterization of sodium optical molasses

Paul D. Lett, P. L. Gould, William D. Phillips

Published Jan 1, 1987• 0 citations

Abstract

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.

DOI: 10.1364/oam.1987.wu3

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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.

Using six optical beams (+ and − directions on three orthogonal axes) tuned slightly to the red of resonance it is possible to cool sodium atoms to a few hundred microkelvin. The cooling occurs because the laser radiation pressure creates a damping force which reduces the atomic velocities to a rms value of ~60 cm/s, determined by random scattering. In this optical molasses, the laser beams propagate in all directions and the light acts as a viscous photon fluid which damps the atomic velocity. The resulting motion of the sodium atoms is diffusive until the atoms reach the edge of the laser beams where they escape. The process of atomic confinement by optical molasses can be modeled as a random walk in a viscous medium with a boundary. For a spherical volume of 0.2 cm3 the resulting confinement time is ~0.1 s for half of the atoms to escape. Direct time-of-flight measurement of the atomic temperature gives ~240 μK, which agrees with predictions of quantum limited cooling by resonance radiation pressure. With confinement times of 0.1 s and temperatures of ~240 μK this method provides a source of atoms sufficiently slow for even the highest resolution optical spectroscopy, collision studies, or optical traps.

By the moving molasses technique we have extracted laser-cooled cesium atoms in a continuous way directly from an optical molasses. The mean launching velocity is precisely tunable from 1 to 12 m/s. The atomic beam has a flux of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{8} \mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}/\mathrm{s}$ at 7 m/s and a longitudinal temperature of $70 \ensuremath{\mu}\mathrm{K},$ which represents the highest flux and lowest velocity spread obtained so far in a continuous beam of cold atoms. These features makes it well suited for atomic fountains. The atomic flux can be slightly increased in a two-dimensional magneto-optical trap operation

(+40%).

A simple model accounts for the observed dependence of the flux with the magnetic-field gradient.

We have studied in detail the Zeeman-tuned slowing of a beam of sodium atoms. We present experimental data that examine details of the slowing by studying the atomic beam both as it slows and after the slowing process has been completed. A Monte Carlo computer simulation reproduces many of the details of our experimental data. We also discuss an analytic model based on the idea that the slowing proceeds adiabatically—that is, such that slowing atoms are always nearly resonant with the laser. This model allows us to give simple physical interpretations to many of the features of our data.

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.

We report the first observation of optically trapped atoms. Sodium atoms cooled below ${10}^{\ensuremath{-}3}$ K in "optical molasses" are captured by a dipole-force optical trap created by a single, strongly focused, Gaussian laser beam tuned several hundred gigahertz below the

{D}_{1}

resonance transition. We estimate that about 500 atoms are confined in a volume of about

{10}^{3}

\ensuremath{\mu}

{\mathrm{m}}^{3}

at a density of

{10}^{11}

-

{10}^{12}

${\mathrm{cm}}^{\ensuremath{-}3}$. Trap lifetimes are limited by background pressure to several seconds. The observed trapping behavior is in good quantitative agreement with theoretical expectations.