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The following article appeared in Journal of Applied Physics 110.12 (2011): 123527 and may be found at http://scitation.aip.org/content/aip/journal/jap/110/12/10.1063/1.3671406, The energy transfer processes between Tb3 and Yb3 ions are studied in Tb-based fluoride single crystals. These are very transparent from the UV to the IR wavelength regions, except for the characteristic absorption lines of the rare-earth ions under study. In contrast with previously reported Tb 3-doped glasses and oxide powders, these crystals containing a high Tb3 concentration present two major advantages for the study of the energy transfer processes in the Tb3-Yb3 ion pair. Firstly, the adverse influence of host defects is minimized with the use of high quality crystals. Secondly, the high Tb 3 concentration guarantees a much higher absorption cross-section of UV light, and consequently these crystals have real potential for practical applications. Photoluminescence spectra in the visible-IR wavelength region demonstrate the existence of efficient down and up conversion processes by crossed excitation and emission characteristics of Tb3 and Yb3 ions. In the down conversion process, Tb3( 5D 4) → 2Yb 3( 2F 5/2), two IR photons are emitted for each Tb3 ion deexcited by second-order energy transfer to two Yb3 ions. The total quantum efficiency of the down conversion process in the fluoride system is shown to increase linearly with the Yb3 concentration, reaching its maximum at the Yb3 solubility limit in the fluoride host. Further, efficient up conversion by two- and three-IR photon absorption is observed., This work has been partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C), 22560316, 2010.

The following article appeared in Journal of Applied Physics 110.6 (2011): 063102 and may be found at http://scitation.aip.org/content/aip/journal/jap/110/6/10.1063/1.3638040, We report on the optical characterization of Yb3+ doped BaMgF4+ nonlinear fluoride crystal grown by the Czochralski technique. Low temperature absorption spectroscopy reveals that Yb3+ incorporates into the matrix at four well differentiated centers. High resolution site selective experiments have been performed to determine the energy level schemes associated with the major Yb3+ centers detected in the system. The fluorescence decay times recorded at 10 K under selective excitation are analyzed for each Yb3+ center. The spectroscopic behavior of the codoped Yb3+:Na+:BaMgF4+ system has been also investigated. Codoping with Na+ eliminates two Yb3+ centers present in the singly doped Yb3+:BaMgF 4+ crystal. The charge compensation mechanisms and site location for Yb3+ in the fluoride matrix are discussed, This work has been supported by Spanish MICINN under Contract Nos. MAT2010-17443 and MAT2009-06580, Comunidad de Madrid under Grant No. PHAMA S2009/MAT-1756 and Universidad Autónoma de Madrid under Contract No. CCG10-UAM/MAT-5290.