Tm-Yb-Tm energy transfers and effect of temperature on the fluorescence intensities in oxyfluoride tellurite compounds

Abstract

Up-conversion processes in PbF2+TeO2+WO3 glass-ceramics doped with Tm3+ and codoped with Tm3+ and Yb3+ ions were investigated under 680 nm excitation. Emission, excitation and time-resolved spectra and decay profiles were measured at temperatures between 12 and 300 K. A strong blue emission centred at 478 nm originating from the 1G4 level of Tm3+ was observed in a doubly doped sample via a two-step energy transfer mechanism, (Tm) 3F4 to (Yb) 2E5/2 energy transfer and a positive feedback (Yb) 2F5/2 to (Tm) 1G4 process, raising the excited Tm3+ ion from the 3F4 to the 1G4 level following the absorption of 680 nm excitation light. The time-resolved emission spectra prove the validity of these energy transfer processes. The optical properties of the excited levels and the energy transfer processes involved in the up-conversion were interpreted using a kinetic model describing the interactions between rare-earth ions in terms of energy migration and luminescence quenching. The microscopic interaction parameter for Tm-Tm intersection in the absence of Yb3+ ions was determined to be 6.1*10-40 cm6 s-1 in the sample having 1.0 mol% TmF3. The same parameter for the energy transfer from Tm to Yb was calculated to be 1.2*10-38 cm6 s-1 in the sample having 0.15 mol% TmF3 and 15 mol% YbF3. No effect of temperature on the emission intensity of the (Tm) 3F4 level was observed although the emission intensities of the (Tm) 1G4 and (Yb) 2F5/2 levels were decreased with increasing temperature. The former decrease accounts for the migration of energy between Yb3+ ions and controls the temperature dependence of the blue-up conversion emission.