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LASER COOLING DUE TO ATOMIC DIPOLE ORIENTATION

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

Published Jul 18, 1988• 2 citations
OpenAlex

Abstract

The temperature of a gas of sodium atoms confined in optical molasses” was originally measured to be -2M0 μK 1, In agreement with the theoretical prediction kBT - Kr/2 2, where r = 2π × 10 MHz is the natural linewidth of the atomic transition used in cooling. More recently, Phillips, et al, 3 have measured temperatures well below the accepted Doppler cooling limit. These results have been confirmed by groups at Stanford and Paris 4

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Annual Meeting Optical Society of America•Jan 1, 1985•1 citation
The temperature of a gas of sodium atoms confined in optical molasses” was originally measured to be -2M0 μK 1, In agreement with the theoretical prediction kBT - Kr/2 2, where r = 2π × 10 MHz is the natural linewidth of the atomic transition used in cooling. More recently, Phillips, et al, 3 have measured temperatures well below the accepted Doppler cooling limit. These results have been confirmed by groups at Stanford and Paris 4
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.
The usual case in laser cooling is that the atomic linewidth ħγ is much greater than the recoil energy ER = ħ2k2/2 m. Considering the recent demonstration of laser cooling to kBT <<ħγ,1 It seems reasonable to reconsider laser cooling where ERħγ. In this case the Doppler shift ħk2/m of a recoil velocity is large compared to the linewidth and an analytic treatment is difficult. We have developed a Monte Carlo simulation of 3-D laser cooling (optical molasses) which applies to any value of ħγ/ER.
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 present a summary of the results of a simple two-level theory of Doppler cooling in optical molasses and contrast it with the recent theories of multilevel, polarization-gradient cooling. The effects of single-photon recoil and of trapping in microscopic optical potential wells are also considered. Experiments are described in which the temperature of sodium atoms released from optical molasses is measured and found to be well below the Doppler-cooling limit. Measurements of the temperature dependence on many experimental parameters are found to be in good qualitative agreement with the new theories of polarization-gradient cooling.
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.