optical molasses


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optical molasses

[′äp·tə·kəl mə′las·əs]
(optics)
A viscous damping force exerted on neutral atoms by a pair of identical oppositely directed lasers tuned at a frequency below an atomic resonance.
A large number of atoms collected and cooled in a small volume at the intersection of the beams of three orthogonal pairs of such lasers.
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Ashkin and Bjorkholm point out that the optical molasses produces what amounts to a new state of matter, an ultra-cooled gas.
"Optical molasses is the standard starting point for just about anyone's laser-cooling experiments," says Chu.
The team also used far less expensive diode lasers -- similar to those in compact-disk players -- first to "catch" cesium atoms for study, and then to generate the cooling "optical molasses." The physicists managed to cage their record cold sample in a magnetic field for about 1 second, establishing another record.
To complete the cooling, they mired the atoms in the "optical molasses" existing at the intersection of six crisscrossing laser beams.
Scientists later realized that they had based their predictions on a simplified model of the sodium atom and that they had not accounted for variations in the laser-induced electric fields within the optical molasses. Complex interactions with those electric fields caused the atoms to slow down, and cool off, more than expected.
He and his colleagues at Bell Laboratories in Holmdel, N.J., used an array of six lasers that converged at a single point in space to create a region they called "optical molasses." The researchers then steered sodium atoms into this space, where they became stuck.
The optical molasses method makes use of the Doppler effect-- the change in the observed frequency of light due to relative motion of the source and observer.
The atoms travel from the optical molasses source region through a region that is used to detect atoms later in the process, and into the magnetically shielded C-field section of the fountain.
The ball of atoms is then launched by differential detuning of the two vertical laser beams to make a moving optical molasses. After the atoms have been accelerated to their launch velocity the molasses laser beams are all detuned to the red in frequency while simultaneously reducing the optical intensity to further cool the launched atom sample.