Sol-Gel-derived Er3+/Yb3+ co-activated silica-alumina xerogels

Several Er3+/Yb3+ codoped silica-alumina monolithic xerogels were prepared with the same concentration of 2000 Er/Si ppm and 6000 Al/Si ppm. The Yb/Si content was varied from 0 to 4000 ppm. Densification of the samples was achieved by thermal treatment in air at 950°C for 120 hours with a heating rate 0.1°C/min. Luminescence spectroscopy were measured as a function of Yb3+ doping concentration upon excitation at 980 nm. Photoluminescence excitation spectroscopy was used to obtain information about the effective excitation efficiency of Er3+ ions by co-doping with Yb3+ ions. The NIR-to visible upconversion luminescence has been investigated upon continuous wave (CW) excitation at 980 nm. The samples exhibit red, green and blue upconversion emission. The dependence of upconversion luminescence intensities on pump power has been evaluated and experimental data were fitted to a power law, I ? P^n. It is found that intensity of upconversion luminescence is increased with increasing Yb3+ doping concentration. Based on the experimental results, a model has been proposed for energy transfer and upconversion emissions, which consist of two and three photons excitation process.