Sub-Doppler cooling

Equation (Cl.4.35) yields two remarkable predictions first, tliat tire sub-Doppler friction coefficient can be a big number compared to since at far detuning Aj /T is a big number and second, tliat a p is independent of tire applied field intensity. This last result contrasts sharjDly witli tire Doppler friction coefficient which is proportional to field intensity up to saturation (see equation (C1.4.24). However, even tliough a p looks impressive, tire range of atomic velocities over which is can operate are restricted by tire condition tliat T lcv. The ratio of tire capture velocities for Doppler versus sub-Doppler cooling is tlierefore only uipi/uj 2 Figure Cl. 4.6 illustrates... 

Figure C 1.4.6. Comparison of capture velocity for Doppler cooling and Tin-periD-lin sub-Doppler cooling. Notice tliat tire slope of tire curves, proportional to tire friction coefficient, is much steeper for tire sub-Doppler mechanism. (After [17].)...

In accordance with the above temperature-scale estimates (Fig. 5.4), it is convenient to consider consecutively the cooling of atoms first to the Doppler limit Td, then sub-Doppler cooling to Tree, and finally subrecoil cooling. Various mechanisms for cooling atoms in light fields of various configurations are considered below in the same sequence. The reader can find more detailed analysis in the book by Metcalf and van der Straten (1999). 

PURELY ROTATIONAL COHERENCE AND SUB-DOPPLER SPECTROSCOPY. Guided by the theoretical decay simulations of Fig. 46, the first unambiguous observation of thermally averaged rotational coherence effects was made for excitation and detection of the S, - S00° band of jet-cooled t-stilbene.47 Observed fluorescence decays are shown in Fig. 47 theory and experiment match very well. The recurrences associated with rotational coherence effects in fluorescence have been observed for a number of other species as well. Among these species are t-stilbene-, 2, t-stilbene-argon complexes,48 and t-stilbene-he-lium complexes.71 The recurrences allow the determination of the excited-state rotational constants to a high degree of accuracy. [For example, for t-stilbene we find j(B + C) to be 0.00854 + 0.00004 cm-1.] The indications are that with currently available temporal resolution, rotational coherence effects should be observable in a multitude of species and should allow the accurate determination of such species excited-state rotational constants. 

SUB-DOPPUIR COOLING In 1988 the NIST-Gaithersburg group made careful measurements of the temperature of atoms laser cooled in optical molasses, cind found temperatures significantly below the Doppler cooling limit 113). The initial measurements on laser cooled sodium atoms gave temperatures of about 40 pK, about six times lower tham the predicted lower limit of 240 pK. 

G. Raithel, S. L. Rolston, R. J. C. Spreeuw, and C. I. Westbrook. I also want to remember and acknowledge Richard N. Watts, who died in November 1996 at the age of 39. His pioneering work on laser cooling with diode lasers, sub-Doppler laser cooling and localization of laser cooled atoms prepeired the way for all the subsequent work on optical lattices. 

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