Decrease in Fluorescence Yield due to Rehydration of Sol-gel Glasses
Yuqi Mao, T. Nguyen Nguyen, and A. Silversmith
The sol-gel method of making glass is an alternative to melt-glass techniques that prepares materials at lower temperatures. Potentially sol-gel glasses can contain higher concentrations of rare-earth (RE) ions without losing the amorphous character of the material. Applications such as phosphors and solid-state lasers, require that strong fluorescence quenching in these materials be addressed.
One mechanism for reduced fluorescence in Tb3+ doped glass is energy transfer from an excited RE ion to a hydroxyl group remaining in the glass after processing steps. After annealing the glasses at high temperatures ~900—1000ºC to remove the residual hydroxyl groups, studies show that for the acid-catalyzed glass samples, water molecules from the surrounding air permeate back into the samples. Within hours after taking the glass out of the furnace after annealing to 900ºC, the fluorescence from the 5D3®7FJ transitions is decreases by at least an order of magnitude. Varying amounts of Tb3+ as well as Al3+, a network modifier, show no difference in the relative rate of decay for the acid-catalyzed glasses. While higher temperatures( ~1000ºC) show more densification of the glass and a decrease in rehydration, the material loses optical clarity with visible foaming on the inside.
In addition to raising the annealing temperature, making the samples through base catalysis show some promise in reducing rehydration of annealed glass samples. Base-catalyzed samples should have large clusters of glassy complex, with RE ions tightly sealed within and inaccessible to the closest hydroxyl groups for the energy transfer. We developed a procedure to produce base-catalyzed samples. This procedure is difficult to control, but preliminary data show that fluorescence quenching of 5D3®7FJ violet/blue fluorescence due to rehydration may occur at a reduced level.
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