Cooperative energy transfer as a probe of clustering in Yb3+ doped fluoroaluminate glasses

Abstract

We report experimental observations of impurity Tm3+ ions upconversion luminescence in fluoroaluminate glasses doped with Yb3+ ions in concentration of 1019–1021 ions/cm3. It was shown that even at extra low concentrations of impurity Tm3+ ions of 1014–2 1017 ions/cm3 there is observable upconversion luminescence of Tm3+ ions. The presence of upconversion luminescence from Tm3+ impurities indicates that Ln3+ ions are clustered in fluoroaluminate glasses. The value of n-parameter of Tm3+ upconversion luminescence band at 475 nm (1G4→3H6) decreases from 3 to 2 as the excitation power density changes from the range of 2.0–6.0 W/cm2 to the range of 1.4–12.4 kW/cm2. The value of n-parameter indicates that predominant mechanism of energy transfer is cooperative energy transfer from excited Yb–Yb* dimers at high excitation power density and three-stage energy transfer from a single Yb3+ ion to impurity Tm3+ ions at low excitation power.