Optical molasses 2734556 204520458 2008-04-09T19:36:14Z Aaraamaa 6856715 /* References */ [[Image:Molasses.gif|350px|thumb|Optical Molasses Schematic]] Optical molasses is a [[laser cooling]] technique that can cool down neutral [[atom]]s 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. ==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. ==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_Effect|Stark Shift]] of atoms in different [[Fine_structure|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. ==References== Laser cooling below the Doppler limit by polarization gradients: simple theoretical models J. Dalibard and C. [[Claude_Cohen-Tannoudji|Cohen-Tannoudji]] JOSA B, Vol. 6, Issue 11, pp. 2023- (1989) [[Category:Atomic, molecular, and optical physics]]