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American Journal of Physics

One-dimensional laser cooling of an atomic beam in a sealed vapor cell

Chris J. Vale, M. R. Walkiewicz, Peter M. Farrell, Robert E Scholten

Published Jan 1, 2002• 3 citations
Publisher

Abstract

We describe a simple experiment that demonstrates one-dimensional laser cooling in a sealed vapor cell. A velocity selective optical pumping scheme was first used to define a collimated beam of atoms within the cell. A particular velocity group of atoms was labeled by optical pumping with one laser and detected by absorption with a second laser. Transverse cooling of this velocity group, which formed an atomic beam between the two laser beams, was then observed by applying a third laser beam, in analogy with transverse cooling of a conventional atomic beam.

DOI
10.1119/1.1419098

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We describe a simple experiment that demonstrates one-dimensional laser cooling in a sealed vapor cell. A velocity selective optical pumping scheme was first used to define a collimated beam of atoms within the cell. A particular velocity group of atoms was labeled by optical pumping with one laser and detected by absorption with a second laser. Transverse cooling of this velocity group, which formed an atomic beam between the two laser beams, was then observed by applying a third laser beam, in analogy with transverse cooling of a conventional atomic beam.
The thermal velocity distribution of a calcium atomic beam was compressed into a narrow velocity distribution with selectable velocities. Subsequently these slow atoms were deflected by an angle of 30° by a one-dimensional optical molasses. In the deflected beam we obtained an estimated flux of several 1010 atoms/s at mean longitudinal velocities near 35 m/s and a velocity width of approximately 13 m/s. The transverse velocities of the deflected atoms were close to the one-dimensional Doppler limit. We have observed saturated absorption signals in the deflected laser-cooled beam. The reduced transit time broadening allowed us to resolve the recoil splitting in saturated absorption.
We report a new cooling scheme used to actively collimate an atomic beam of 85Rb^{85}\mathrm{Rb}85Rb with one-dimensional optical molasses. The atoms' transverse velocity is damped by a standing wave of circularly polarized light in a magnetic field of \ensuremath{\sim}20 \ensuremath{\mu}T to 2 cm/s (rms), well below the Doppler velocity vD{v}_{D}vD​=(7\ensuremath{\Elzxh}\ensuremath{\gamma}/20M)1/2{)}^{1/2})1/2\ensuremath{\sim}10 cm/s for one-dimensional optical molasses. The minimum measured velocity is limited only by the experimental geometry. We have developed a model in which the mixing by the field and optical pumping combine to form a cycle that cools below the Doppler limit, in agreement with our measurements.
We have realized efficient transverse cooling of an Indium atomic beam by combining optical pumping with a closed cycle UV laser cooling transition at 325.6 nm. The transverse velocity of the atomic beam is reduced to 13.5+/-3.8 cm/s, well below the Doppler cooling limit. The fraction of laser-cooled In atoms is enhanced to 12+/-3 % by optical pumping in the present experiment. It can be scaled up to approach 100% efficiency in cooling, providing high brightness atomic beams for further applications. Our results establish In on the map of elements suitable for applications involving laser cooling.
Laser cooled atomic gases and atomic beams are widely studied samples in experimental research in atomic and optical physics. For the application of ultra cold gases as model systems for e.g. quantum many particle systems, the atomic species is not very important. Thus this field is dominated by alkaline, earthalkaline elements which are easily accessible with conventional laser sources and have convenient closed cooling transition. On the otherhand, laser cooled atoms may also be interesting for technological applications, for instance for the creation of novel materials by atomic nanofabrication (ANF). There it will be important to use technologically relevant materials. As an example, using group III atoms of the periodical table in ANF may open a route to generate fully 3D structured composite materials. The minimal requirement in such an ANF experiment is the collimation of an atomic beam which is accessible by one dimensional laser cooling. In this dissertation, I describe transverse laser cooling of an Indium atomic beam. For efficient laser cooling on a cycling transition, I have built a tunable, continuous-wave coherent ultraviolet source at 326 nm based on frequency tripling. For this purpose, two independent high power Yb-doped fiber amplifiers for the generation of the fundamental radiation at 977 nm have been constructed. I have observed sub-Doppler transverse laser cooling of an Indium atomic beam on a cycling transition of In by introducing a polarization gradient in the linear-perpendicular-linear configuration. The transverse velocity spread of a laser-cooled In atomic beam at full width at half maximum was achieved to be 13.5 ± 3.8 cm/s yielding a full divergence of only 0.48 ± 0.13 mrad. In addition, nonlinear spectroscopy of a 3-level, Lambda-type level system driven by a pump and a probe beam has been investigated in order to understand the absorption line shapes used as a frequency reference in a previous two-color spectroscopy experiment. For the analysis of this atomic system, I have applied a density matrix theory providing an excellent basis for understanding the observed line shapes.
We present a new laser-cooling scheme based on velocity-selective optical pumping of atoms into a nonabsorbing coherent superposition of states. This method has allowed us to achieve transverse cooling of metastable 4He^{4}\mathrm{He}4He atoms to a temperature of 2 \ensuremath{\mu}K, lower than both the usual Doppler cooling limit (23 \ensuremath{\mu}K) and the one-photon recoil energy (4 \ensuremath{\mu}K). The corresponding de Broglie wavelength (1.4 \ensuremath{\mu}m) is larger than the atomic-transition optical wavelength.