Spectroscopic studies of radiative transitions and upconversion characteristics of Er^3+ ions in simple pseudoternary fluoride glasses MF_n–BaF_2–YF_3 (M: Zr, Hf, Al, Sc, Ga, In, or Zn)

Abstract

Optical properties of Er³⁺ -doped fluoride glasses in the pseudoternary systems MF_n–BaF₂–YF₃, where M is Zr, Hf, Al, Sc, Ga, In, or Zn, were investigated, and the variation of the local structure was discussed. Of the three Judd–Ofelt Ω_t parameters (t = 2, 4, 6) that dominate the f–f transition probabilities of Er³⁺ ions, Ω₂ increased as the cationic field strength of network-forming Mⁿ⁺ ions was increased. Ω₆ showed a correlation with the Er—F bond length. The intensity of the ⁴S_3/2→⁴I_15/2 green upconversion luminescence at 0.55 μm induced by 0.80-μm laser pumping varied with the kind of MF_n; the InF₃-based glass showed the largest intensity, and the AlF₃-based glass the lowest. The quantum efficiency and nonradiative decay rates of the ⁴S_3/2 level changed with Mⁿ⁺ and were not strongly dependent on the cutoff phonon energy of the host. The transition probability of the excitation process at 0.80-μm absorption and that of 0.55-μm emission can play a more important role in the upconversion intensity than the nonradiative loss in the present fluoride glasses with a low multiphonon decay rate.