Optical properties of transparent glass-ceramics containing Er3+-doped sodium lutetium fluoride nanocrystals

Transparent glass-ceramics containing Er3+-doped sodium lutetium fluoride nanocrystals for photonic applications have been synthesized. Glass transition temperature, softening temperature, and crystallization temperature were estimated by dilatometry and differential thermal analysis. Proper heat treatments were selected to crystallize lutetium fluoride nanocrystals. X-ray diffraction analysis was carried out to identify the crystalline phase and the crystal size. HRTEM indicates that the base glass is phase separated in droplets enriched in Lu, Na, F, and also Er ions. The thermal treatment induces the crystallization inside the droplets. The optical characterization, which includes absorption and steady-state and time-resolved emission spectroscopy under one- and two-photon excitation, shows the differences between the phase-separated base glass and its corresponding glass-ceramic. The reduction of the Judd-Ofelt parameter (2) together with the increase of the fluorescence lifetimes as compared to the glass sample confirms the presence of Er3+ ions in a crystalline environment in the glass-ceramic samples. Moreover, an enhancement of the green and red up-converted emissions (as well as the weak blue emission) is observed in the glass-ceramic, indicating the Er3+ incorporation into the nanocrystals. The possible excitation mechanisms responsible for this up-conversion luminescence are discussed on the basis of lifetime measurement results.