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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Local internal and bulk optical cooling in Nd-doped crystals and nanocrystalline powders

Ángel J. García-Adeva, Rolindes B. Balda, M. Al-Saleh, Joaquin R. Fernandez

Published Feb 11, 2010• 3 citations
Publisher

Abstract

The first experimental demonstration of local internal and bulk optical cooling in samples of Nd-doped KPb2Cl5 crystals and Nd-doped KPb2Cl5 nanocrystalline powders upon laser excitation between the 4F3/2 and 4F5/2 manifolds is reported. The possibility of controlling the dynamics of the bulk optical cooling process by adequately tuning the excitation wavelength is also demonstrated.

DOI
10.1117/12.842116

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The enhanced laser cooling performance of rare-earth-ion-doped nanocrystalline powders is predicted, compared to the bulk material, using Yb3+:Y2O3{\mathrm{Yb}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{O}}_{3}Yb3+:Y2​O3​ as the model material. This is achieved by enhancing the off-resonance, phonon-assisted absorption, which is proportional to the three factors considered in this paper: the dopant concentration, the pumping field energy, and the excitation coefficient. Using the energy transfer theory for concentration quenching, the optimum concentration corresponding to the maximum cooling power is found to be considerably 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 (and it is shown that the reabsorption of the fluorescence is negligible). Using molecular dynamics simulations, the phonon density of states (DOS) of the nanopowder is calculated, and found to have broadened modes, and 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. It is finally concluded that these three enhancement mechanisms are essentially to increase the population of the three participating carriers (electron, photon, and phonon) in the interacting volume, and this also points out directions for enhancing laser cooling performance in bulk materials.
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