Mechanisms and Concentration Dependence of Upconversion Fluorescence in Er³⁺ and Tm³⁺ Codoped Tellurite Glasses

We investigate the upconversion luminescence of Er³⁺ and Tm³⁺ codoped tellurite glasses under both the 975 and 800nm excitations. By Tm³⁺ codoping, the Er³⁺ green emission corresponding to the (⁴S_3/2, ²H_11/2) → ⁴I_15/2 transitions was quenched, while the red emission corresponding to the ⁴ F_9/2 → ⁴I_15/2 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 800nm excitations, respectively. The results were explained considering the influence of energy transfers between these two active ions.