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

We have investigated the decay processes of the Er:4S3/2 and F39/2 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:4S3/2 and F49/2 states are discussed on the basis of the Er concentration dependence of the fluorescence lifetime. The quantum efficiencies of emission of the S43/2 and F49/2 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 S43/2 state is much smaller than that of the F49/2 state. The multiphonon relaxation is a dominant nonradiative decay process of the F49/2 state. A dominant nonradiative decay process of the S43/2 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 I49/2→4I13/2 and F49/2←4I11/2 transitions. We also show that the upconversion from 800 to 550 nm is due to excited-state absorption.