Trivalent lanthanides, also called rare-earth ions (REIs), show light emissions in almost all the near infrared and visible spectrum. Such luminescent properties are often related to potential applications in a variety of fields since sharp emission lines, meta-stable states, the ability of converting long/short-wavelengths (up/down-conversion process), energy transfer processes (resonant and non-resonant) and high quantum efficiency make them appropriate for the manufacture of many optical devices.
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This work was financially supported by the Brazilian agencies FAPESP, CNPq and CEPOF/INOF. V.A.G. Rivera extends thanks FAPESP for financial support (project 2009/08978-4 and 2011/21293-0) that allowed my post-doctoral and my gratitude to Dr. Luiz Antonio Nunes of the Instituto de F?sica de S?o Carlos-University S?o Paulo-Brazil and the Dr. Yannick Ledemi and the Dr. Younnes Messaddeq of the Centre d?Optique, Photonique et laser-University Laval-Canada for the discussions on this issue.