Abstract

Laser cooling in standing waves is related to the study of coherent optical transients. Transients in the u-component of the optical Bloch vector lead to a simple and unified physical picture of cooling at both low saturation parameter (optical molasses) and high saturation parameter (stimulated cooling). Optical molasses is shown to result from a free induction decay transient depending on the coherence relaxation time T2, while stimulated cooling is an incoherent effect of the population relaxation time T1. The sign, tuning, and saturation characteristics of each are attributed to specific transient phenomena. The model offers a bare-state physical picture of laser cooling which complements the dressed-state "Sisyphus" model. Phase-interrupting processes which make T2 < 2T1 reduce the trap potential by a factor of T2/2T1 and lead to a cooling force proportional to T2–2T1 which has not appeared in previous theories. The new term dominates both optical molasses and stimulated cooling when T2 «T1.

© 1990 Optical Society of America