Abstract
Glass-ceramics containing NaYF 4 nanocrystals were prepared by heat-treatment from oxyfluoride silicate-based glass doped with Ho 3+ 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.
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