In a simple one-dimensional version, an optical molasses is made with two counterpropagating laser beams, the frequency of which is tuned slightly below an atomic absorption resonance. As a result, a motion of an atom (or ion) in the direction of one of the beams will lead to a Doppler shift so that the absorption rate for the counterpropagating beam is increased, whereas the absorption rate for the opposite laser beam is reduced. Effectively there is a dissipative light force which is always directed opposite to the motion and therefore serves to reduce that motion.
A three-dimensional optical molasses can be made of six laser beams, propagating e.g. in the +X, −X, +Y, −Y, +Z, and −Z directions. Such an arrangement can reduce the motion in any direction of space. It can therefore effectively reduce the temperature of an atomic (or ion) cloud. The standard temperature limit is the Doppler limit, but there are techniques to reach even substantially lower temperatures.
It is also possible to use different frequencies of the counterpropagating beams, effectively generating a moving optical molasses which tends to bring the particle velocities toward a certain value. Through the Doppler shift, particles moving with that velocity experience equal frequencies of all the waves.
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|||S. Chu et al., “Three-dimensional viscous confinement and cooling of atoms by resonance radiation pressure”, Phys. Rev. Lett. 55 (1), 48 (1985), doi:10.1103/PhysRevLett.55.48|
|||P. D. Lett et al., “Optical molasses”, J. Opt. Soc. Am. B 6 (11), 2084 (1989), doi:10.1364/JOSAB.6.002084|