Effect of Temperature on Upconversion Luminescence in $Yb^{3+}//Tb^{3+}$ Co-Doped Germanate Glass

In the article effect of temperature on the cooperative energy transfer in germanate glass co-doped with $Yb^{3+}//Tb^{3+}$ under 976 nm laser diode pumping was investigated. The optimization of $Tb^{3+}$ concentration on the upconversion luminescence was determined. Strong luminescence at 489, 543, 586, 621 nm corresponding to $\text{}^{5}D_{4} \rightarrow \text{}^{7}F_{J}$ (J = 6,4,3) transitions and luminescence at 381, 415, 435 nm resulting from $\text{}^{5}D_{3}, \text{}^{5}G_6 \rightarrow \text{}^{7}F_{J}$ (J=6,5,4) transitions were presented. The highest upconversion emission intensity was obtained in glass co-doped with $0.7Yb_{2}O_{3}//0.7Tb_{2}O_{3}$ (mol%). The effect of temperature on the luminescent properties of germanate glass in the range of 5-250°C indicates the presence of competing phenomena: an increase in the effective absorption cross-section of $Yb^{3+}$ ions donor as a function of temperature and migration of energy between pairs of ions $Yb^{3+}-Yb^{3+}$ and of multiphonon excitation levels $\text{}^{7}F_{J}$.