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Optics and Spectroscopy

Deep Raman cooling of alkaline-earth atoms

Yu. D. Anuchina, Vladimir S. Ivanov, Yu. Rozhdestvensky

Published May 1, 2015• 0 citations
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Abstract

We have analyzed deep Raman cooling of alkaline-earth atoms with two stable and two excited states in a field of three traveling light waves. It has been shown that the excitation of this system by Raman pulses applied between lowest states of the system can be used to deeply cool its atoms to a temperature that is on the order of or even lower than the temperature determined by the recoil energy.

DOI
10.1134/s0030400x15050021

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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•May 1, 2013•0 citations
We have analyzed deep Raman cooling of alkaline-earth atoms with two stable and two excited states in a field of three traveling light waves. It has been shown that the excitation of this system by Raman pulses applied between lowest states of the system can be used to deeply cool its atoms to a temperature that is on the order of or even lower than the temperature determined by the recoil energy.
Analysis of optical excitation of a double Λ system with three lower and two excited states (so-called M atom) by the field of four travelling optical waves is carried out. It is shown that excitation of such a system by Raman π pulses can be used for deep cooling of M atoms to a temperature much lower than the temperature controlled by the recoil energy. The application of the proposed scheme considerably simplifies practical implementation of deep Raman cooling, which considerably extends the range of application of this technique.
Cooling of atoms below the temperature determined by the recoil energy is proposed on the basis of the Raman excitation of three-level atoms by the field of standing waves with a relative spatial shift. The advantage of this cooling mechanism is a weak sensitivity to the shape and duration of light pulses used for the transfer of population under Raman excitation. It is shown that the effectiveness of such cooling increases sharply when multizone excitation is used and already several interaction zones are enough for deep transverse cooling of an atomic beam to a temperature significantly lower than the recoil energy.
Quench cooling is a promising technique to reach ultra-cold temperatures in alkaline-earth atoms by Doppler cooling on ultra-narrow transitions. The principles of quench cooling are derived from an effective two-level system with a linewidth adjustable by the quenching laser. A tunable linewidth reconciles the contradictory requirements of a fast cooling rate and a high velocity selectivity at high and low temperatures, respectively. In this paper, we investigate the efficiency of quench cooling in alkaline-earth systems. We present a one-dimensional analytical description of the quenching process. Cooling and trapping in three dimensions is studied with semi-classical Monte Carlo simulations. Our results for magnesium indicate a loading efficiency of up to 40% of pre-cooled atoms at 2 mK into a QuenchMOT. Final temperatures of 9µ K and an increase in phase-space density by almost five orders of magnitude are observed in the simulations.
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 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 $123\ensuremath{\mu}\mathrm{K}.$ The efficiency of this cooling scheme and the main loss mechanisms are analyzed.
We report the implementation of a two-dimensional Raman cooling scheme using sequential excitations along the orthogonal axes. Using square pulses, we have cooled a cloud of ultracold cesium atoms down to an rms velocity spread of 0.39(5) recoil velocities, corresponding to an effective transverse temperature of 30nK30\phantom{\rule{0.3em}{0ex}}\mathrm{nK}30nK (0.15Trec)(0.15{T}_{\mathrm{rec}})(0.15Trec​). This technique can be useful to improve cold-atom atomic clocks and is particularly relevant for clocks in microgravity.
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.