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Annual Meeting Optical Society of America

Observation of atoms laser-cooled below the Doppler limit

William D. Phillips, Paul D. Lett, Richard N. Watts, Christoph Ira Westbrook, P. L. Gould, H. Metcalf

Published Jan 1, 1988• 29 citations
Publisher

Abstract

By measuring the ballistic motion of sodium atoms released from optical molasses1 we determine their temperature to be as low as 43 ± 20 μK. Four different measurement techniques are used, giving temperatures ranging from 43 to 80 μK. These observations are inconsistent with the accepted theory of laser cooling,2 which sets a lower limit on the temperature of kBT = ħγ/2. This Doppler cooling limit is achieved for a detuning of Δ = – γ/2. For sodium cooled on the D2 line, γ = 2π × 10 MHz, and the Doppler cooling limit is 240 μK. We have measured the detuning dependence of the temperature and found it to be in strong disagreement with theoretical predictions. The temperature is minimized for |δ| much larger than γ/2 and is always below the cooling limit except at small detunings. These results are remarkable and unexpected; they raise important questions about the adequacy of present theories of laser cooling.

DOI
10.1364/oam.1988.fa2

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By measuring the ballistic motion of sodium atoms released from optical molasses1 we determine their temperature to be as low as 43 ± 20 μK. Four different measurement techniques are used, giving temperatures ranging from 43 to 80 μK. These observations are inconsistent with the accepted theory of laser cooling,2 which sets a lower limit on the temperature of kBT = ħγ/2. This Doppler cooling limit is achieved for a detuning of Δ = – γ/2. For sodium cooled on the D2 line, γ = 2π × 10 MHz, and the Doppler cooling limit is 240 μK. We have measured the detuning dependence of the temperature and found it to be in strong disagreement with theoretical predictions. The temperature is minimized for |δ| much larger than γ/2 and is always below the cooling limit except at small detunings. These results are remarkable and unexpected; they raise important questions about the adequacy of present theories of laser cooling.
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 on the realization of sub-Doppler laser cooling of sodium atoms in gray molasses using the D1{D}_{1}D1​ optical transition $(3s\phantom{\rule{0.16em}{0ex}}^{2}S_{1/2}\ensuremath{\rightarrow}3p\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2})$ at 589.8 nm. The technique is applied to samples containing $3\ifmmode\times\else\texttimes\fi{}{10}^{9}$ atoms, previously cooled to $350\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ in a magneto-optical trap, and it leads to temperatures as low as $9\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and phase-space densities in the range of ${10}^{\ensuremath{-}4}$. The capture efficiency of the gray molasses is larger than 2/3, and we observe no density-dependent heating for densities up to ${10}^{11}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.
We have measured the spring constant, friction parameter, and temperature of the motion of caesium atoms in a magneto-optical trap. The results are very different from those predicted by models based on the scattering force, which have been used up to now. We show that the behaviour can be understood in terms of the sub-Doppler cooling mechanisms which have been observed in molasses, involving atomic motion through a polarization gradient. These mechanisms can operate in a trap because the atoms congregate in a region of low magnetic field. The lowest temperature measured was (30 ± 15) μK.
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