Up-conversion luminescence properties and energy transfer of Er³⁺/Yb³⁺ co-doped oxyfluoride glass ceramic containing CaF₂ nano-crystals

The Er³⁺/Yb³⁺ co-doped transparent oxyfluoride glass-ceramics containing CaF₂ nano-crystals were successfully prepared. After heat treatments, transmission electron microscopy (TEM) images showed that CaF₂ nano-crystals of 20-30 nm in diameter precipitated uniformly in the glass matrix. Comparing with the host glass, high efficiency upconversion luminescence of Er³⁺ at 540 nm and 658 nm was observed in the glass ceramics under the excitation of 980 nm. Moreover, the size of the precipitated nano-crystals can be controlled by heat-treatment temperature and time. With the increase of the nano-crystal size, the intensity of the red emission increased more rapidly than that of the green emission. The energy transfer process of Er³⁺ and Yb³⁺ was convinced and the possible mechanism of Er³⁺ up-conversion was discussed.