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arXiv:physics/0303042 · Physical Review A

Two-photon Doppler cooling of alkaline-earth-metal and ytterbium atoms

Wictor C. Magno, Reinaldo L. Cavasso Filho, Flávio C. Cruz

Published Apr 22, 2003• 31 citations
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Abstract

The possibility of laser cooling of alkaline-earth-metal atoms and ytterbium atoms using a two-photon transition is analyzed. We consider a ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{S}_{0}$ transition with excitation in near resonance with the ${}^{1}{P}_{1}$ level. This greatly increases the two-photon transition rate, allowing an effective transfer of momentum. The experimental implementation of this technique is discussed and we show that for calcium, for example, two-photon cooling can be used to achieve a Doppler limit of $123\ensuremath{\mu}\mathrm{K}.$ The efficiency of this cooling scheme and the main loss mechanisms are analyzed.

DOI
10.1103/physreva.67.043407

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A two-photon mechanism for cooling atoms below the Doppler temperature is analyzed. We consider the magnesium ladder system $(3{s}^{2})^{1}S_{0}\ensuremath{\rightarrow}(3s3p)^{1}P_{1}$ at $285.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ followed by the $(3s3p)^{1}P_{1}\ensuremath{\rightarrow}(3s3d)^{1}D_{2}$ transition at $880.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. For the ladder system quantum coherence effects may become important. Combined with the basic two-level Doppler cooling process this allows for reduction of the atomic sample temperature by more than a factor of 10 over a broad frequency range. First experimental evidence for the two-photon cooling process is presented and compared to model calculations. Agreement between theory and experiment is excellent. In addition, by properly choosing the Rabi frequencies of the two optical transitions a velocity independent atomic dark state is observed.

Physical Review A•Nov 16, 2005•27 citations
The possibility of laser cooling of alkaline-earth-metal atoms and ytterbium atoms using a two-photon transition is analyzed. We consider a 1S0\ensuremath−1S0{}^{1}{S}_{0}{\ensuremath{-}}^{1}{S}_{0}1S0​\ensuremath−1S0​ transition with excitation in near resonance with the 1P1{}^{1}{P}_{1}1P1​ level. This greatly increases the two-photon transition rate, allowing an effective transfer of momentum. The experimental implementation of this technique is discussed and we show that for calcium, for example, two-photon cooling can be used to achieve a Doppler limit of 123\ensuremathμK.123\ensuremath{\mu}\mathrm{K}.123\ensuremathμK. The efficiency of this cooling scheme and the main loss mechanisms are analyzed.
A new possibility of laser cooling of Calcium atoms using a two-photon transition is analyzed We investigate the possibility of Doppler cooling of Calcium using its two-photon (4s 2 ) 1 S0 - (4s5s) 1 S0 transition, with excitation in near resonance with the (4s4p) 1 P1 level. The two-photon absorption of l aser beams at 423 and 1 034 nm greatly increases the two-photon rate, allowing an effective transfer of momentum. The ex perimental i mplementation o f t his technique is discussed a nd we show that two-photon cooling can be used to achieve a temperature limit of about 1/7 of the onephoton Doppler limit reached in a conventional MOT.
We demonstrate cooling of calcium atoms using a two-photon transition from the 1S0{}^{1}{S}_{0}1S0​ ground state to the upper 4s5s1S04s5s\phantom{\rule{3.33333pt}{0ex}}{}^{1}{S}_{0}4s5s1S0​ state via the 1P1{}^{1}{P}_{1}1P1​ intermediate state. We achieve temperatures as low as $260\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{K}$ in a magnetooptical trap (MOT), well below the Doppler limit (TD=0.8mK{T}_{\text{D}}=0.8\phantom{\rule{0.16em}{0ex}}\mathrm{m}\mathrm{K}TD​=0.8mK) of the 1P1{}^{1}{P}_{1}1P1​ state. We characterize the temperature, lifetime, and confinement of the MOT over a range of experimental parameters, observing no reduction in lifetime due to coupling to the higher state. We perform theoretical simulations of the cooling scheme and observe good agreement with the experimental results. The two-photon cooling scheme presented in this work provides an alternative to the standard Doppler cooling applied to alkaline-earth atoms, based on a sequence of two magnetooptical traps. The advantages of our scheme are the possibility of varying the effective linewidth of the 1P1{}^{1}{P}_{1}1P1​ state, a higher transfer efficiency (close to 100100%100), and a more straightforward experimental implementation.
Abstract We analyze laser cooling of two-electron atoms with three-level cascade transitions, involving the usual 1 S 0 – 1 P 1 cooling transition coupled to either 1 S 0 or 1 D 2 states. We numerically calculate minimum temperatures for the most abundant even isotopes of group II atoms and ytterbium (which lack hyperfine structure), derive general analytical expressions from a perturbative approach, and discuss the experimental implementation of the technique. Considered as an alternative 2nd stage technique to intercombination line cooling, three-level cooling is clearly advantageous for light elements, such as Mg and Ca, while for all elements provides much higher capture ranges, increasing the transfer efficiency from the 1st stage cooling, with faster cooling times and sub-Doppler temperatures.
Summary form only given. We consider laser cooling of free atoms with simultaneous two-color excitation of three-level cascade transitions [1], [2], [3], [4], [5], [6] finding theoretically that temperatures below the Doppler limits associated with each one of the individual transitions are obtained. We use standard laser cooling analysis [7], [8] and the dressed state picture in order to identify different cooling regions, and determining optimum parameters for guiding further experiments. These new “sub-Doppler” limits are reached at the two-photon resonance where a significant increase of the damping coefficient and decrease of the diffusion take place. In the dressed state picture, this point corresponds to a crossing of the narrow and a (partially) dark state.Figure 1 shows a plot of temperature (normalized to the Doppler limit for cooling only on the lower transition) as function of single-photon detunings, for the 1S0-1P1-1D2 transition of Magnesium, in which the lower 1S0-1P1 transition is the usual cooling one (λ1 = 2πc/ω1 = 285 nm, λ2 = 2πc/ω2 = 881 nm, γ1 = 2π 78.8 MHz, γ2 = 2π 2.2 MHz). A minimum temperature of 38 μK is predicted. This temperature is lower than the Doppler temperature of the lower and upper transitions, TD = γ/k = 1.9 mK and 49 μK respectively. Due to the asymmetric absorption profile of the dressed atom, there is a remarkable increase in the damping coefficient (α) associated with the narrow transition, in comparison with the broad transition (two-level cooling). The cooling and damping times, inversely proportional to α, are reduced by the same amount. Since the curve for the diffusion coefficient (D) has the same shape as the curve for the absorption coefficient or the population of the intermediate state, D is also reduced with respect to the broad resonance, We analyze situations previously implemented experimentally in alkaline-Earth atoms using cascade transitions, and also discuss prospects for Aand V-type three-level systems.
Doppler laser cooling of a three-level ladder system using two near-resonant laser fields is analyzed in the case of the intermediate level being metastable while the upper level is short lived. Analytical as well as numerical results, e.g., obtainable scattering rates and achievable temperatures are presented. When appropriate, comparisons with two-level single-photon Doppler laser cooling is made. These results are relevant to recent experimental Doppler laser cooling investigations addressing intercombination lines in alkaline-earth-metal atoms and quadrupole transitions in alkaline-earth-metal ions.
A two-photon mechanism for cooling atoms below the Doppler temperature is analyzed. We consider the magnesium ladder system $(3{s}^{2})^{1}S_{0}\ensuremath{\rightarrow}(3s3p)^{1}P_{1}$ at 285.2nm285.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}285.2nm followed by the $(3s3p)^{1}P_{1}\ensuremath{\rightarrow}(3s3d)^{1}D_{2}$ transition at 880.7nm880.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}880.7nm. For the ladder system quantum coherence effects may become important. Combined with the basic two-level Doppler cooling process this allows for reduction of the atomic sample temperature by more than a factor of 10 over a broad frequency range. First experimental evidence for the two-photon cooling process is presented and compared to model calculations. Agreement between theory and experiment is excellent. In addition, by properly choosing the Rabi frequencies of the two optical transitions a velocity independent atomic dark state is observed.