Optical molasses
An optical molasses consists of 3 pairs of counter-propagating orthogonally polarized laser beams intersecting in the region where the atoms are present.
The main difference between an optical molasses and a magneto-optical trap (MOT) is the absence of magnetic field in the former.
When laser cooling was proposed in 1975, a theoretical limit on the lowest possible temperature was predicted.
The first experimental realization of optical molasses was achieved in 1985 by Chu et al. at AT&T Bell Laboratories.
By temporarily switching off the laser beams for a fixed time interval, the authors firstly measured the average kinetic energy of the atoms by a time-of-flight technique.
The fraction of atoms that left the region while it was in the dark was measured by comparing the brightness of the fluorescence before and after the turnoff.
Then velocity distribution and temperature were measured by estimating the dependence of this fraction on the light-off time.
The kinetic temperature they obtained was T ≈ 240 μK, not very different from the Doppler cooling limit in the two-level approximation.
Experiments at the National Institute of Standards and Technology in Gaithersburg found the temperature of cooled atoms to be well below the theoretical limit.
[3] In 1988, Lett et al.[3] directed sodium atoms through an optical molasses and found the temperatures to be as low as ~40 μk, 6 times lower than the expected 240 μk Doppler cooling limit.
These unexpected observations led to the development of more sophisticated models[5] of laser cooling that took into account the Zeeman and hyperfine sublevels of the atomic structure.
The dynamics of optical pumping between these sublevels allow the cooling of atoms below the Doppler limit.
The best explanation of the phenomenon of optical molasses is based on the principle of polarization gradient cooling.
[6] For one-dimensional optical molasses: Suppose two laser beams approach an atom from opposite directions.
Counterpropagating beams of circularly polarized light cause a standing wave, where the light polarization is linear but the direction rotates along the direction of the beams at a very fast rate.
though practically the limit is a few times this value because of the extreme sensitivity to external magnetic fields in this cooling scheme.
Atoms typically reach temperatures on the order of microkelvins, as compared to the doppler limit
The one-dimensional optical molasses can be extended to three dimensions with six counter-propagating laser beams.
The temperature obtained varies with the configuration of the laser polarization and are all higher than the theoretical estimate.
In 3D experiments, the transverse nature of light leads to the limitation that there will always be polarization gradients.
An optical molasses slows down the atoms but does not provide any trapping force to confine them spatially.
A magneto-optical trap employs a 3-dimensional optical molasses along with a spatially varying magnetic field to slow down and confine the atoms.
