Er3+/Yb3+ co-activated silica-alumina monolithic xerogels

Monolithic silica xerogels doped with different concentrations of Er3+, Yb3+ and Al3+ were prepared by sol gel route. The starting composition was a mixture of tetramethylorthosilicate, methanol, deionized water and nitric acid in the molar ratio 0.06:0.35:0.55:0.04, respectively. The rare-earth ions and the aluminium were introduced as Er(NO3)3?5(H2O), Yb(NO3)3?5(H2O) and Al(NO3)3?9(H2O). Densification was achieved by thermal treatment in air at 950 oC for 120 h with a heating rate of 0.1 oC/min. The Yb3+ ion is an useful co-dopant for Er3+ mainly due the three properties: a) Yb3+ is a two-level system and its excited level 2F5/2 can transfer energy with good efficiency to the 4I11/2 level of Er3+; b) the ionic radius of Yb3+ is close to that of Er3+, so that it is possible to surround each Er3+ with several Yb3+ ions; c) should clustering occur, it may well be not between Er3+ ions only, but between Er3+ and Yb3+ ions: this as well the previous property make easier and more efficient the energy transfer between the two ion species. Moreover, it is well known that addition of alumina is effective in dispersing rare-earth ions in silica glasses and, in particular, increases the fluorescence lifetime of the metastable level 4I13/2 of Er3+ ions and can increase the flatness of the 4I13/2 ? 4I15/2 emission band. In this paper we study the luminescence properties of the 4I13/2 ? 4I15/2 emission band of Er3+ as a function of the Al/Er/Yb concentration and we pay particular attention to the concentration effects and self-absorption mechanism. NIR-to-Visible and green-to-blue up-conversion processes upon CW excitation at 980 and 514.5 nm are also investigated. Raman spectroscopy is used to monitor the degree of densification of the glasses and the residual OH content.