Abstract
In a two-level system under traveling wave excitation, the cooling force is bounded by the spontaneous force, given by the product of the photon momentum and the excited-state decay rate. This is because the force in such a scheme originates from absorption of directed photons followed by emission of spontaneous photons in random directions. In the case of a standing wave excitation, this limit no longer holds. The atoms can, in principle, absorb photons from one direction, while emitting coherent photons in the other direction via stimulated emission. This process, known as stimulated cooling, requires symmetry breaking, leading to effects such as Doppleron cooling, which occur for specific combinations of velocities, rabi frequencies, and detunings.1 For small variations in velocity, for example, the force can turn from heating to cooling. As a result, the standing wave cooling is not convenient for slowing down a thermal beam with a broad velocity spread.
© 1996 Optical Society of America
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