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Physical Review A

Expansion and cooling of a bright rubidium three-dimensional optical molasses

Laurence Pruvost, Isabelle Serre, H. T. Duong, Joshua Jortner

Published Apr 14, 2000• 64 citations
Publisher

Abstract

We have studied the expansion of a bright rubidium three-dimensional optical molasses realized with an intense laser light. In this regime, in addition to the laser friction force, the atoms strongly repel each other due to the radiation trapping force, which is analogous to the Coulomb force. The experimental observations show that the cloud volume increases linearly in time and that, simultaneously, the cloud temperature decreases following the law $(1+t/\ensuremath{\tau}{)}^{\ensuremath{-}4/3}.$ These time evolutions are explained using a physical picture of Coulomb explosion in a viscous medium.

DOI
10.1103/physreva.61.053408

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We have studied the expansion of a bright rubidium three-dimensional optical molasses realized with an intense laser light. In this regime, in addition to the laser friction force, the atoms strongly repel each other due to the radiation trapping force, which is analogous to the Coulomb force. The experimental observations show that the cloud volume increases linearly in time and that, simultaneously, the cloud temperature decreases following the law (1+t/\ensuremathτ)\ensuremath−4/3.(1+t/\ensuremath{\tau}{)}^{\ensuremath{-}4/3}.(1+t/\ensuremathτ)\ensuremath−4/3. These time evolutions are explained using a physical picture of Coulomb explosion in a viscous medium.
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.
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.
Researchers in Germany and France have bound neutral atoms in regular two- and three-dimensional lattices induced by the interference patterns of several laser beams. The work builds on the techniques of magnetooptical trapping and sub-Doppler laser cooling and on previous work in which atoms were bound in onedimensional standing waves of light. The fields of light in these techniques are called optical molasses, because atoms within them experience viscous forces, which cool the atoms to temperatures on the order of microkelvins. In the new, “crystalline” molasses, the standing waves produce a stable, regular array of potential wells—a microscopic “egg carton”— and the atoms become localized at the bottoms of the wells.
In this thesis, the effect of radiation trapping on the photon statistics of the light scattered from optical molasses is experimentally investigated.It is demonstrated that the intensity correlation function is sensitive to the presence of radiation trapping at an on-resonance optical depth, OD, as low as 0.1.It is also shown that it is possible by introducing a mild anisotropy in the trapped cloud to change the number density of a trapped sample of laser-cooled atoms in a dilute magnetooptical trap by over an order of magnitude, from 1.9 × 10 9 /cm 3 to 1.6 × 10 8 /cm 3 , while keeping the temperature constant.
We have measured the low-density characteristics of a magneto-optical trap for both isotopes of rubidium. Trapped-atom temperatures are measured by the time-of-flight technique and are similar to temperatures in optical molasses. They are below the Doppler limit in most cases and increase linearly with the ratio of laser intensity to laser detuning. A slight difference between the two isotopes is observed. The data agree well with three-dimensional, multistate, semiclassical simulations. Spring constants of the trap are inferred from careful measurements of the size of the trapped-atom cloud. The spring constant is seen to increase with intensity at low intensity, becoming independent of intensity at high intensity. This trend is consistent with sub-Doppler cooling mechanisms.
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 D1{D}_{1}D1​ resonance transition. We estimate that about 500 atoms are confined in a volume of about 103{10}^{3}103 \ensuremath{\mu}m3{\mathrm{m}}^{3}m3 at a density of 1011{10}^{11}1011-1012{10}^{12}1012 ${\mathrm{cm}}^{\ensuremath{-}3}$. Trap lifetimes are limited by background pressure to several seconds. The observed trapping behavior is in good quantitative agreement with theoretical expectations.