Host dependent frequency upconversion of Er3+-doped oxyfluoride germanate glasses

Er3+-doped oxyfluoride germanate glasses have been synthesized by the conventional melting and quenching method. The Judd–Ofelt intensity parameters were calculated based on the Judd–Ofelt theory and absorption spectra measurements. With the substitution of PbF2 for PbO, the image parameter decreases, while the image parameter increases. These change trends indicate that fluoride anions come to coordinate erbium cations and the covalency of the Er–O bond decreases. Structural and thermal stability properties were obtained by Raman spectra and differential thermal analysis, indicating that PbF2 plays an important role in the formation of glass network and has an important influence on the maximum phonon energy and thermal stability of host glasses. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions 2H11/2→4I15/2, 4S3/2→4I15/2, and 4F9/2→4I15/2, respectively, were simultaneously observed at room temperature. With increasing PbF2 content, the intensity of red (657 nm) emissions increases significantly, while that of the green (525 and 546 nm) emission increases slightly. The results indicate that PbF2 has more influence on the red (657 nm) emission than the green (525 and 546 nm) emissions in oxyfluoride germanate glasses. The possible upconversion luminescence mechanisms have also been estimated and discussed.