Mechanisms and Concentration Dependence of Upconversion Fluorescence in Er 3+ and Tm 3+ Codoped Tellurite Glasses

We investigate the upconversion luminescence of Er3+ and Tm3+ codoped tellurite glasses under both the 975 and 800 nm excitations. By Tm3+ codoping, the Er3+ green emission corresponding to the (4S3/2, 2H11/2)→ 4I15/2 transitions was quenched, while the red emission corresponding to the 4F9/2→ 4I15/2 transition was selectively sensitized. The red emission has a maximum in the range where the ratio of Er3+- to Tm3+-content is about two and its fluorescence intensity becomes 1.5 and 5 times larger at the maximum than those in the absence of Tm3+ for 975 and 800 nm excitations, respectively. The results were explained considering the influence of energy transfers between these two active ions.