Time-resolved fluorescence line narrowing in Yb3+-doped fluoroindate glasses

Fluorescence line narrowing measurements have been carried out between the lowest Stark components of the F-2(5/2)-->F-2(7/2) transition of Yb3+ ions in fluoroindate glasses. At low concentration (0.15 mol % of Yb3+) the luminescence spectrum consists in a narrow band which repeats the exciting profile, so the migration processes between Yb3+ ions are not important at this concentration. However, at higher concentrations (0.75 or 2.25 mol %) the luminescence spectrum contains moreover a broad band produced by other ions nonexcited initially (background fluorescence). A model is proposed to describe the temporal evolution of the narrow band fluorescence and the shape of the background fluorescence. A very good agreement with the experimental results is found taking into account a dipole-quadrupole (S=8) nonradiative energy transfer process assisted by a phonon.