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arXiv:1401.1828 · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Laser cooling of dense atomic gases by collisional redistribution of radiation and spectroscopy of molecular dimers in a dense buffer gas environment

Anne Saß, Ralf Forge, Stavros Christopoulos, Katharina Knicker, Peter Moroshkin, Martin Weitz

Published Feb 19, 2014• 3 citations
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Abstract

We study laser cooling of atomic gases by collisional redistribution of fluorescence. In a high pressure buffer gas regime, frequent collisions perturb the energy levels of the alkali atoms; which enables the absorption of a far red detuned irradiated laser beam. Subsequent spontaneous decay occurs close to the unperturbed resonance frequency, leading to a cooling of the dense gas mixture by redistribution of fluorescence. Thermal defection spectroscopy indicates large relative temperature changes down to and even below room temperature starting from an initial cell temperature near 700K. We are currently performing a detailed analysis of the temperature distribution in the cell. As we expect this cooling technique to work also for molecular-noble gas mixtures, we also present initial spectroscopic experiments on alkali-dimers in a dense buffer gas surrounding.

DOI
10.1117/12.2035630

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We study laser cooling of atomic gases by collisional redistribution of fluorescence. In a high pressure buffer gas regime, frequent collisions perturb the energy levels of the alkali atoms; which enables the absorption of a far red detuned irradiated laser beam. Subsequent spontaneous decay occurs close to the unperturbed resonance frequency, leading to a cooling of the dense gas mixture by redistribution of fluorescence. Thermal defection spectroscopy indicates large relative temperature changes down to and even below room temperature starting from an initial cell temperature near 700K. We are currently performing a detailed analysis of the temperature distribution in the cell. As we expect this cooling technique to work also for molecular-noble gas mixtures, we also present initial spectroscopic experiments on alkali-dimers in a dense buffer gas surrounding.
Laser cooling via collisional redistribution of fluorescence utilizes dense mixtures of alkali metals with noble buffer gases. Typical pressure values of the buffer gas are of the order of a few hundred bar, ensuring a frequent number of collisions between the two atomic species. The energy levels of the alkali atoms are thus perturbed so that excitation using far red-detuned laser light becomes feasible. Energy is then extracted via spontaneous emission occuring close to the unperturbed atomic resonances. Optimization of the cooling effect strongly depends on excitation conditions, namely the choice of detuning of the cooling beam, which in turn is dependent on the used pressure. We here report on spectroscopy measurements of atomic rubidium under high pressure buffer gas conditions. While thermal deflection spectroscopy has been previously used to measure the temperature change of the laser-cooled gas, an alternative approach for temperature measurements utilizing the Kennard-Stepanov relation is also investigated. We here exhibit that the collisionally thermalized atomic resonances well follow this thermodynamic scaling, allowing for temperature extraction of the dense gas mixture.
We describe experiments on the laser cooling of both helium-rubidium and argon-rubidium gas mixtures by collisional redistribution of radiation. Frequent alkali-noble gas collisions in the ultradense gas, with typically 200 bar of noble buffer gas pressure, shift a highly red detuned optical beam into resonance with a rubidium Dline transition, while spontaneous decay occurs close to the unshifted atomic resonance frequency. The technique allows for the laser cooling of macroscopic ensembles of gas atoms. The use of helium as a buffer gas leads to smaller temperature changes within the gas volume due to the high thermal conductivity of this buffer gas, as compared to the heavier argon noble gas, while the heat transfer within the cell is improved.
We report on experiments investigating laser cooling of atomic gases by collisional redistribution of radiation, a technique applicable to dense mixtures of alkali metals with noble gases. Thermal deflection spectroscopy is one of the methods used to measure the temperature change of the laser-cooled gas. In this work we describe experiments focusing on a different technique for precise determination of the local temperature achieved by the cooling within the gas cell. We investigate the Kennard-Stepanov relation, a thermodynamic, Boltzmann-type scaling between the absorption and emission spectral profiles of an absorber, which applies in many liquid state dye solutions as well as in semiconductor systems. To this end, absorption and emission spectra of rubidium atoms and dimers in dense argon buffer gas environment have been recorded. We demonstrate experimentally that the Kennard-Stepanov relation between absorption and emission spectra is well fulfilled for the collisionally broadened atomic and molecular transitions of the system, which allows for the extraction of the thermodynamic temperature.
We describe measurements demonstrating laser cooling of an atomic gas by means of collisional redistribution of radiation. The experiment uses rubidium atoms in the presence of several hundred bar of argon buffer gas pressure. Frequent collisions in the dense gas transiently shift a far-red detuned optical field into resonance, while spontaneous emission occurs close to the unperturbed atomic transition frequency. Evidence for the cooling is obtained via both thermographic imaging and thermographic deflection spectroscopy. The cooled gas has a density above 1021 atoms/cm3, yielding evidence for the laser cooling of a macroscopic ensemble of gas atoms.
Laser cooling by collisional redistribution of radiation has been successfully applied in the past for cooling dense atomic gases. Here we report on progress of work aiming at the demonstration of redistribution laser cooling in a molecular gas. The candidate molecule strontium monohydride is produced by laser ablation of strontium dihydride in a pressurized noble gas atmosphere. The composition of the ablation plasma plume is analyzed by measuring its emission spectrum. The dynamics of SrH molecular density following the ablation laser pulse is studied as a function of the buffer gas pressure and the laser intensity.