Upconversion Luminescence of Er3+ Ions in Transparent Germanate Glass Ceramics Containing CaF₂ Nanocrystals

The visible upconversion and near-infrared luminescence of Er³⁺ ions in germanate glass ceramics containing CaF₂ nanocrystals are investigated. The nanocrystals are characterized by x-ray diffraction (XRD) and transmission electron microscopy, showing their mean sizes less than 20 nm. High transmittance of the glass ceramics is displayed by absorption spectra. The upconversion luminescence intensity in the glass ceramics increases significantly with increasing temperature. Both the shifts of the XRD peaks and the Stark-split shown in the luminescence spectra indicate the entrance of the Er³⁺ ions into the CaF₂ nanocrystals, which is confirmed by a Judd-Ofelt analysis. Possible mechanisms of the upconversion luminescence are analyzed.