Analysis of energy transfer processes in Yb³⁺-Tb³⁺ co-doped, low-silica calcium aluminosilicate glasses

Energy transfer processes in Yb³⁺-Tb³⁺ co-doped, low-silica calcium aluminosilicate glasses were analyzed. Luminescence and time-resolved measurements were used to study upconversion processes, such as Yb-Tb cooperative sensitization, Yb-Yb cooperative luminescence, and Yb-Tb cross relaxation. The quantum cross relaxation efficiency was evaluated as a function on the Yb³⁺ concentration, and the maximum estimated value was approximately 51. In addition, the intensity of the upconversion luminescence from the Tb³⁺:⁵D₄ level decreased by two orders of magnitude comparing the value at room temperature with that at 123 K. As a consequence, Yb-Yb cooperative luminescence around 500 nm became comparable with the intensity of upconversion from the Tb³⁺:⁵D ₄ level. Furthermore, a dependence of the upconversion kinetics luminescence on temperature was observed. The upconversion rise time was constant and equal to 65 s for temperatures between 296 to 473 K and decreased from 65 to 19 μs, without variation in the decay part, when the temperature was lowered from 296 to 123 K. These results were explained by a phonon-assisted cooperative sensitization process for the population of the Tb ³⁺:⁵D₄ level.