Investigation of sub-Doppler cooling in an ytterbium magneto-optical trap

We report experimental evidence of Sisyphus cooling in atoms with a ¹S₀ ground state. Since $J=0,$ any cooling mechanism which requires multiple sublevels can only occur in the isotopes which have nuclear spin I≠0. Ytterbium has seven stable isotopes and offers a unique system in which we can study cooling on F=→01 (168,170,172,174,176Yb), F=1/→23/2 (¹⁷¹Yb), and F=5/→27/2 (¹⁷³Yb), depending on the selection of isotope. We have trapped each of the seven stable isotopes of ytterbium in a magneto-optical trap (MOT) using the strong ¹S₀-¹P₁ transition, and transferred them into a second MOT which uses the much narrower ¹S₀-³P₁ intercombination transition. We have measured the temperature of isotopes ¹⁷¹Yb, ¹⁷³Yb, and ¹⁷⁴Yb in the ¹S₀-³P₁ MOT, as a function of the intensity and detuning of the trapping laser. The temperature of ¹⁷⁴Yb was found to increase more rapidly with intensity than predicted by Doppler cooling theory, in agreement with earlier work on alkaline-earth atoms. In the odd isotopes the temperature was found to decrease with increasing angular momentum, as observed in earlier experiments and three-dimensional simulations.