Optical molasses

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Optical molasses is a laser cooling technique that can cool down neutral atoms to temperatures colder than a magneto-optical trap (MOT). An optical molasses consists of 3 pairs of counter-propagating circularly polarized laser beams intersecting in the region where the atoms are present. The main difference between optical molasses and a MOT is the absence of magnetic field in the former. While a typical Sodium MOT can cool atoms down to 300μK, optical molasses can cool the atoms down to 40μK, an order of magnitude colder.

[edit] History

When laser cooling and MOTs became common in the late 1970s, a theoretical limit on the lowest possible temperature was predicted. Known as the Doppler Limit, this was given by the lowest possible temperature attainable in a MOT considering the cooling of atoms by doppler cooling and the heating of atoms due to the scattering of laser photons

Experiments at the National Institute of Standards and Technology, Gaithersburg, found the temperature of cooled atoms to be well below the theoretical limit. Initially, it was a surprise to theorists, until the full explanation came out.

[edit] Theory

The best explanation of the phenomenon of optical molasses is based on the principle of polarization gradient cooling. Counterpropagating beams of circularly polarized light cause a standing wave, where the light polarization depends on the spatial location. The AC Stark Shift of atoms in different magnetic sub-levels is also spatially dependent. The basic idea is that atoms moving with a velocity climb a polarization gradient hill, thereby losing their velocity. At the top of the hill, atoms are resonant with the other molasses beams, absorb a photon and decay into a lower energy magnetic sub-level, thereby having shed some of their velocity.

[edit] References

Laser cooling below the Doppler limit by polarization gradients: simple theoretical models J. Dalibard and C. Cohen-Tannoudji JOSA B, Vol. 6, Issue 11, pp. 2023- (1989)