Probing magneto-optic trap dynamics through weak excitation of a coupled narrow-linewidth transition

Abstract

Ytterbium ~Yb, Z570) and other alkaline-earth-like atoms possess coupled two-level-like transitions ~see Fig. 1! whose widely different natural widths @1,2# support multiple magneto-optic trapping opportunities. Specifically, the strong and nearly closed @3# S0P1 transitions are well suited, due to their large scattering rates, to cooling atomic beams @2,4– 6,9,10# and loading magneto-optic traps ~MOTs! from thermal or slowed sources @4–6,9,10#. In contrast, and due to their narrow linewidths, the spin-forbidden S0P1 transitions support MOTs with ultralow limiting temperatures and potentially high spatial densities @5,7#, and are useful for high-resolution spectroscopic studies @8–10#. Employing these two types of transitions in complementary cooling, trapping, and spectroscopic roles may ultimately provide new routes to quantum degeneracy @5,7# and high-precision optical frequency standards @1,8–10#. A staged cooling experiment using first the S0P1 and then the S0P1 transitions in strontium ~Sr! has already led to record phasespace densities in a MOT @5#. Additionally, optical spectroscopy of the S0P1 magnesium ~Mg! @9# ~calcium ~Ca! @10#! transition in S0P1 MOTs has produced fractional frequency stabilities exceeding ~approaching! that which can be obtained with atomic-beam experiments using the same species. In this Rapid Communication, we present observations, in a steady-state 398.8-nm(6s)S0-(6s6p) P1 Yb MOT, of probe fluorescence spectra induced by excitation of the Yb(6s)S0-(6s6p) P1 555.6-nm transition. We find that the Zeeman structure of the P1 excited state is completely resolved and that peak widths and splittings provide diagnostic information about the atomic cloud size, location relative to the trap magnetic field zero @11#, and potentially, the atomic velocity distribution @10#. Use of an intercombination transition to perform in situ temperature measurements may provide an alternative approach to trapdestructive time-of-flight techniques @10,12#. Since a detailed description of the apparatus has been given previously @4#, we only review the relevant features of the trapping experiment here. Approximately 10 Yb atoms, loaded with a s Zeeman slower, are held in a S0P1 Yb MOT. For the power levels used in this experiment, the trap lifetime t is limited primarily by radiative branching from the P1 state to t;400 msec @4#. Trap axial