The red emission in two and three steps up-conversion process in a doped erbium SiO2–TiO2 sol–gel powder

The spectra of the two and three photon up-conversion process in SiO2–TiO2-doped erbium is studied and the nature of the red emission discussed. In both processes, the red emission (4F9/2→4I15/2) is measured, however, when pumped directly at 490 nm such emission is not observed. The two-photon up-conversion has to take place first in order to observe the red emission; meanwhile for the case of the three-photon up-conversion it is not necessary for this process to occur first. The fluorescence decay of the green up-converted and direct pumped green emissions were measured and consistently with our results a non-exponential decay is observed.