A study on microstructure and luminescent properties of oxyfluoride silicate glass-ceramics with (Ho~(3+),Yb~(3+)):NaYF_4 crystallites

摘要

Glass-ceramics containing NaYF_4 nanocrystals were prepared by heat-treatment from oxyfluoride silicate-based glass doped with Ho3* and Yb~(3+) ions. The formation of crystalline fluoride phase was confirmed by X-ray diffraction and transmission electron microscopy. Absorption and emission spectra revealed that a fraction of Ho~(3+) and Yb~(3+) ions is incorporated into the NaYF_4 ordered lattice influencing spectroscopic features of glass-ceramics in comparison with those of precursor glass. Green up-conversion emission (545 nm) originating in the ~5S_2 level in glass-ceramics and up-converted red emission (650 nm) originating in the ~5F_5 level in as-melted glass were observed under optical pumping into Yb~(3+) absorption band and analyzed. Although both emissions in both materials are achieved by two-photon excitations, the relation between green and red emission intensity in glass-ceramics and glass implies that processes relevant to up-conversion phenomena are different. Based on a careful analysis of relaxation dynamics of Ho~(3+) and Yb~(3+) excited states, the mechanisms involved in conversion of the infrared radiation into the visible emission in these materials are proposed and discussed.