Time dependence and energy-transfer mechanisms in Tm3+, Ho3+ and Tm3+–Ho3+ co-doped alkali niobium tellurite glasses sensitized by Yb3+

Glass samples with the composition (mol%) 80TeO2–10Nb2O5–5K2O–5Li2O, stable against crystallization, were prepared containing Yb3+, Tm3+ and Ho3+. The energy transfer and energy back transfer mechanisms in samples containing 5% Yb3+–5% Tm3+ and 5% Yb3+–5% Tm3+–0.5% Ho3+ were estimated by measuring the absorption and fluorescence spectra together with the time dependence of the Yb3+ 2F5/2 excited state. A good fit for the luminescence time evolution was obtained with the Yokota–Tanimotoʹs diffusion-limited model. The up-conversion fluorescence was also studied in 5% Yb–5% Tm, 5% Yb–0.5% Ho and 5% Yb–5% Tm–0.5% Ho tellurite glasses under laser excitation at 975 nm. Strong emission was observed from 1G4 and 3F2 Tm3+ energy levels in all samples. The 5S2 Ho3+ emission was observed only in Yb3+Ho3+ samples being completely quenched in Yb3+/Tm3+/Tm3+ samples.