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Enhanced Laser Cooling of Ion-Doped Nanopowders

[+] Author Affiliations
Xiulin Ruan, Massoud Kaviany

University of Michigan

Paper No. IMECE2005-81939, pp. 615-632; 18 pages
doi:10.1115/IMECE2005-81939
From:
  • ASME 2005 International Mechanical Engineering Congress and Exposition
  • Heat Transfer, Part B
  • Orlando, Florida, USA, November 5 – 11, 2005
  • Conference Sponsors: Heat Transfer Division
  • ISBN: 0-7918-4222-3 | eISBN: 0-7918-3769-6
  • Copyright © 2005 by ASME

abstract

Enhanced laser cooling performance of rare-earth ion doped nanocrystalline powders is predicted, using Yb3+ :Y2 O3 as the model material. This is achieved by enhancing the anti-Stokes off-resonance absorption, which is proportional to the three factors considered in this paper: dopant concentration, pumping field energy, and anti-Stokes transition rate. The concept of the optimum dopant concentration for cooling is proposed based on the fact that higher concentration increases absorption while decreases quantum efficiency. Using the concentration quenching theory of energy transfer, the optimum concentration, which gives the maximum cooling power, is found to be larger than the currently used value, suggesting noticeable enhancement effects for laser cooling. The pumping field energy is enhanced in random nanopowders compared with bulk crystals under the same irradiation, due to the multiple scattering of photons. Photons are thus localized in the medium and do not propagate through, increasing the photon absorption of the pumping beam. This also contributes significantly to laser cooling enhancement. Using molecular dynamics simulations, the phonon density of states (DOS) of the nanopowder is calculated, and found to have extended, small tails at low and high frequencies. The second-order electronic transition rate for the anti-Stokes luminescence is calculated using the Fermi golden rule, which includes the influence of this phonon DOS, and is shown to have enhancement effects on the laser cooling efficiency using nanopowders. Finally, it is concluded that these three enhancement mechanisms are exactly equivalent to increasing the number of the three participating carriers (electron, photon, and phonon) in the interacting volume.

Copyright © 2005 by ASME
Topics: Laser cooling

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