In this paper we present the investigation of the energy transfer efficiency between Tb3+ and Yb3+ ions in silica-hafnia waveguides. Cooperative energy transfer between these two ions allows to cut one 488 nm photon in two 980 nm photons and could have important applications in improving the performance of photovoltaic solar cells. Previous works revealed that for a given concentration of donors (Tb3+, increasing the number of acceptors (Yb3+) located near to the Tb3+ ion can increase the Tb-Yb transfer probability. However, when increasing the density of active ions, some detrimental effects due to cross-relaxation mechanisms become relevant. On the basis of this observation the sample doping was chosen keeping constant the molar ratio [Yb]/[Tb] = 4 and the total rare earths contents were [Tb + Yb]/[Si + Hf] = 5%, 7%, 9%. The choice of the matrix is another crucial point to obtain an efficient down conversion processes with rare earth ions. To this respect a 70SiO(2)-30HfO(2) waveguide composition was chosen. The comparison between the glass and the glass-ceramic structures demonstrated that the latter is more efficient since it combines the good optical properties of glasses with the optimal spectroscopic properties of crystals activated by luminescent species. A maximum transfer efficiency of 55% was found for the highest rare earth doping concentration
Abstract. Pure and europium doped silica xerogels were annealed at 1050~ to obtain full densification, and at 1300~ to induce crystallization. Raman spectroscopy, time resolved selective luminescence and lifetime measurements were performed on glassy and crystallized samples. We discuss the differences between the Raman spectra of the xerogel annealed at 1050~ and those of a commercial silica. The typical Raman structures of ot-crystobalite are evident for the 1300~ annealed samples, but a glassy phase coexists, indicating an incomplete crystallization. Fluorescence measurements give information on the environment of the Eu 3+ ions in the glassy and crystallized sample.
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