Luminescence and upconversion processes in ${\mathrm{Er}}^{3+}$-doped tellurite glasses

This work presents a spectroscopic investigation of tellurite glasses with the composition 65TeO₂-15Li₂O-20ZnO doped with different concentrations of Er₂O₃. Optical absorption and emission spectroscopy were performed at room temperature to determine emission properties of Er³⁺ in the visible and near-infrared regions. The samples presented intense green emission due to the ${}_{2}H_{11/2}$ $\to$ ${}_{4}I_{15/2}$ and ${}_{4}S_{3/2}$ $\to$ ${}_{4}I_{15/2}$ transitions. Emission intensities at 992 and 1550 nm exhibit a relative increase with increasing doping concentration. Luminescence decay curves for the ${}_{4}S_{3/2}$ $\to$ ${}_{4}I_{15/2}$ transition follows a non-exponential behavior, while the ${}_{4}I_{11/2}$ $\to$ ${}_{4}I_{15/2}$ and ${}_{4}I_{13/2}$ $\to$ ${}_{4}I_{15/2}$ transitions presented simple exponential behavior with high lifetime values. Under 975 nm excitation, upconversion luminescences in the green and red regions are observed, with a relative increase for red emission as function of doping concentration. The possible mechanisms considering multi-phonon relaxation, energy transfer and cross-relaxation processes were discussed for green and red emissions under excitations at 488 and 975 nm. The square dependence of green emission on the excitation power indicates two photons contribution to the upconversion emission. The full width at half-maximum values of emission at 1550 nm increased with increasing Er³⁺ concentration, showing the potential of the studied material as infrared amplifier.