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Physical Review A

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

Reina H. Maruyama, R. H. Wynar, Michael V. Romalis, A. Andalkar, Matthew D. Swallows, C. E. Pearson, E. Norval Fortson

Published Jul 25, 2003• 75 citations
Publisher

Abstract

We report experimental evidence of Sisyphus cooling in atoms with a ${}^{1}{S}_{0}$ ground state. Since $J=0,$ any cooling mechanism which requires multiple sublevels can only occur in the isotopes which have nuclear spin $I\ensuremath{\ne}0.$ Ytterbium has seven stable isotopes and offers a unique system in which we can study cooling on $F=\stackrel{\ensuremath{\rightarrow}}{0}1$ ${(}^{168,170,172,174,176}\mathrm{Yb}),$ $F=1/\stackrel{\ensuremath{\rightarrow}}{2}3/2$ ${(}^{171}\mathrm{Yb}),$ and $F=5/\stackrel{\ensuremath{\rightarrow}}{2}7/2$ ${(}^{173}\mathrm{Yb}),$ 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 ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}$ transition, and transferred them into a second MOT which uses the much narrower ${}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}$ intercombination transition. We have measured the temperature of isotopes ${}^{171}\mathrm{Yb},$ ${}^{173}\mathrm{Yb},$ and ${}^{174}\mathrm{Yb}$ in the ${}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}$ MOT, as a function of the intensity and detuning of the trapping laser. The temperature of ${}^{174}\mathrm{Yb}$ 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.

DOI
10.1103/physreva.68.011403

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We report experimental evidence of Sisyphus cooling in atoms with a 1S0{}^{1}{S}_{0}1S0​ ground state. Since J=0,J=0,J=0, any cooling mechanism which requires multiple sublevels can only occur in the isotopes which have nuclear spin I\ensuremath≠0.I\ensuremath{\ne}0.I\ensuremath=0. Ytterbium has seven stable isotopes and offers a unique system in which we can study cooling on F=0\ensuremath→1F=\stackrel{\ensuremath{\rightarrow}}{0}1F=0\ensuremath→​1 (168,170,172,174,176Yb),{(}^{168,170,172,174,176}\mathrm{Yb}),(168,170,172,174,176Yb), F=1/2\ensuremath→3/2F=1/\stackrel{\ensuremath{\rightarrow}}{2}3/2F=1/2\ensuremath→​3/2 (171Yb),{(}^{171}\mathrm{Yb}),(171Yb), and F=5/2\ensuremath→7/2F=5/\stackrel{\ensuremath{\rightarrow}}{2}7/2F=5/2\ensuremath→​7/2 (173Yb),{(}^{173}\mathrm{Yb}),(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\ensuremath−1P1{}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}1S0​\ensuremath−1P1​ transition, and transferred them into a second MOT which uses the much narrower 1S0\ensuremath−3P1{}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}1S0​\ensuremath−3P1​ intercombination transition. We have measured the temperature of isotopes 171Yb,{}^{171}\mathrm{Yb},171Yb, 173Yb,{}^{173}\mathrm{Yb},173Yb, and 174Yb{}^{174}\mathrm{Yb}174Yb in the 1S0\ensuremath−3P1{}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}1S0​\ensuremath−3P1​ MOT, as a function of the intensity and detuning of the trapping laser. The temperature of 174Yb{}^{174}\mathrm{Yb}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.
We report on the sub-Doppler laser cooling of neutral Yb171 and Yb173 in a magneto-optical trap (MOT) using the S10−P11 transition at 398.9 nm. We use two independent means to estimate the temperature of the atomic cloud for several of the Yb isotopes. The two methods of MOT cloud imaging and release-and-recapture show consistency with one another. Temperatures below 400 and 200 μK are recorded for Yb171 and Yb173, respectively, while ∼1 mK is measured for both Yb172 and Yb174. By comparison, the associated one-dimensional Doppler cooling temperature limit is 694 μK. The sub-Doppler cooling of the I=1/2Yb171 isotope in a σ+−σ− light-field trap adds further evidence that the Sisyphus cooling mechanism is occurring in such three-dimensional MOTs.
We report the magneto-optical trapping of ytterbium atoms using the ${6s}^{2}{}^{1}{S}_{0}\ensuremath{-}6s6p{}^{3}{P}_{1}$ intercombination transition. The magneto-optical trap is continuously loaded from an atomic beam decelerated by a Zeeman-tuned atomic-beam slower operated at the ${6s}^{2}{}^{1}{S}_{0}\ensuremath{-}6s6p{}^{1}{P}_{1}$ transition. Among seven stable isotopes of ytterbium, six isotopes of170Yb,\mathrm{of}{ }^{170}\mathrm{Y}\mathrm{b},of170Yb, 171Yb,{}^{171}\mathrm{Yb},171Yb, 172Yb,{}^{172}\mathrm{Yb},172Yb, 173Yb,{}^{173}\mathrm{Yb},173Yb, 174Yb,{}^{174}\mathrm{Yb},174Yb, and 176Yb{}^{176}\mathrm{Yb}176Yb are successfully trapped. The number and density of the trapped atoms are about 108{10}^{8}108 and ${10}^{11} {\mathrm{cm}}^{\ensuremath{-}3},$ respectively. The temperature of atoms is estimated to be about $20 \ensuremath{\mu}\mathrm{K}$ from the time-of-flight measurement. The decay time of the trapped atoms after stopping the loading of atoms is about 3 s.
Alkaline-earth-metal atoms present an ideal platform for exploring magneto-optic trap (MOT) dynamics, enabling unique and definitive tests of laser cooling and trapping mechanisms. We have measured the trapping beam intensity, detuning, magnetic-field gradient, trap density, and lifetime dependence of the spring constant $\ensuremath{\kappa}$ and damping coefficient $\ensuremath{\alpha}$ for a ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}$ 88Sr{}^{88}\mathrm{Sr}88Sr MOT by fitting the oscillatory response of the atom cloud to a step-function force. We find that the observed behavior of $\ensuremath{\kappa}$ and $\ensuremath{\alpha}$ provide a unified and consistent picture of trap dynamics that agrees with Doppler cooling theory at the level of 1010%.10 Additionally, we demonstrate that the trapped atom temperature can be determined directly from measured value of $\ensuremath{\kappa}$ and the trap size, in excellent agreement with free-expansion temperature measurements. However, the experimentally determined temperature is much higher than Doppler cooling theory, implying significant additional heating mechanisms.
We demonstrate launching of laser-cooled Yb atoms in a cold atomic fountain. Atoms in a collimated thermal beam are first cooled and captured in a magneto-optical trap (MOT) operating on the strongly allowed ${{}^{1}S}_{0}\ensuremath{\rightarrow}{{}^{1}P}_{1}$ transition at 399 nm (blue line). They are then transferred to a MOT on the weakly allowed ${{}^{1}S}_{0}\ensuremath{\rightarrow}{{}^{3}P}_{1}$ transition at 556 nm (green line). Cold atoms from the green MOT are launched against gravity at a velocity of around 2.5 m/s using a pair of green beams. We trap more than 107{10}^{7}107 atoms in the blue MOT and transfer up to 7070%70 into the green MOT. The temperature for the odd isotope 171Yb^{171}\mathrm{Yb}171Yb is  1mK~1 \mathrm{mK} 1mK in the blue MOT, and reduces by a factor of 40 in the green MOT.
We present a detailed investigation of strontium magneto-optical trap (MOT) dynamics. Relevant physical quantities in the trap, such as temperature, atom number and density, and loss channels and lifetime, are explored with respect to various trap parameters. By studying the oscillatory response of a two-level 1S0–1P1 88Sr MOT, we firmly establish the laser cooling dynamics predicted by Doppler theory. Measurements of the MOT temperature, however, deviate severely from Doppler theory predictions, implying significant additional heating mechanisms. To explore the feasibility of attaining quantum degenerate alkaline-earth samples via evaporative cooling, we also present the first experimental demonstration of magnetically trapped metastable 88Sr. Furthermore, motivated by the goal of establishing the fermionic isotope 87Sr as one of the highest-quality, neutral-atom-based optical frequency standards, we present a preliminary study of sub-Doppler cooling in a 87Sr MOT. A dual-isotope (87Sr and 88Sr) MOT is also demonstrated.
We have studied the temperatures of several neutral Yb isotopes in a magneto-optical trap (MOT) using the1S0-1P1transition at 398.9 nm. We use a compact MOT with a titanium vacuum chamber and a high degree of othogonality between the beams. Minimum temperatures of 170 μK and 80 μK are recorded for171Yb, and173Yb, respectively, while 0.95 mK is measured for172Yb. By comparison the 1D Doppler cooling temperature limit is 694 μK. The results augur well for future lattice clocks that use Yb. We comment on the significance of sub-Doppler cooling of atoms with ground-state total angular momentum F =1/2, as is the case for171Yb.