Red-to-green up-conversion in Er-doped SiO2 and SiO2–TiO2 sol–gel silicate glasses

Monolithic Er-doped SiO2–TiO2 binary glasses of high optical quality were used in an investigation of the effects of different annealing conditions and titanium content on fluorescence yields and decay times of the 4S3/2 level of Er3+. In addition, the characteristics of green upconverted fluorescence from the 4S3/2 level via excitation with red laser light (655 nm) were studied. Energy transfer up-conversion involving two ions in the 4I11/2 state was determined to be the dominant up-conversion mechanism. The glasses with TiO2 showed enhanced up-conversion and required lower annealing temperatures for the up-conversion to be observed when compared to SiO2 glasses doped with Al.