Red to blue upconversion in Tm-doped sol–gel silicate glasses

In thulium-doped glasses derived from the hydrolysis of orthosilicates we worked with different annealing conditions so as to enhance the fluorescence yield. In addition, we observed blue upconverted fluorescence from both the 1 G 4 and 1 D 2 levels of thulium when the glasses were excited with red laser light. Two-step absorption mechanisms with different intermediate levels are responsible for populating the two emitting levels.