Nonradiative decay processes and mechanisms of frequency upconversion of Er³⁺ in ZrF₄-BaF₂-LaF₃ glass

We have investigated the decay processes of the Er:⁴S_3/2 and ⁴F_9/2 states and the mechanisms of frequency upconversion under 800 nm excitation of Er³⁺ in ternary ZrF₄-BaF₂-LaF₃ glasses. The decay processes of the Er:⁴S_3/2 and ⁴F_9/2 states are discussed on the basis of the Er concentration dependence of the fluorescence lifetime. The quantum efficiencies of emission of the ⁴S_3/2 and ⁴F_9/2 states are 73% and 25%, respectively. The difference in the quantum efficiencies between these states can be explained from the difference in the multiphonon decay rates: the multiphonon decay rate of the ⁴S_3/2 state is much smaller than that of the ⁴F_9/2 state. The multiphonon relaxation is a dominant nonradiative decay process of the ⁴F_9/2 state. A dominant nonradiative decay process of the ⁴S_3/2 state is self-quenching at high Er concentrations. The upconversion mechanisms under 800 nm excitation are also investigated from the dependence of upconversion luminescence on excitation wavelengths (800 and 980 nm). excitation laser power, and Er concentration. The mechanism of upconversion from 800 to 660 nm is mainly due to the energy transfer between the ⁴I_9/2→4I_13/2 and ⁴F_9/2←4I_11/2 transitions. We also show that the upconversion from 800 to 550 nm is due to excited-state absorption.