Nanocrystallization in Yb~(3+)-doped oxyfluoride glasses for laser cooling

摘要

Glass-ceramics are composite materials consisting of crystals which are controllably grown within a glass matrix usually by applying an appropriate heat treatment. They possess outstanding optical properties with applications in solid state lasers, optical amplifiers, and now, laser induced cooling. For laser cooling, the material should exhibit specific properties like low phonon energy environment around the lanthanide ions, low background losses, high transparency and high photoluminescence quantum yield. In the present study, oxyfluoride glasses and ultra-transparent nano glass-ceramics doped with different concentrations (2, 5, 8, 12, 16 and 20 mol %) of Yb~(3+) ions have been prepared by conventional melt-quenching and subsequent thermal treatments at different temperatures, respectively. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements have been performed to characterize the thermal properties of the glass and the structural changes in the glass-ceramics, respectively. The XRD patterns confirm the growth of β-PbF_2 nanocrystals as well as progressive incorporation of Yb~(3+) ions. This enhances the Yb~(3+) ion emission intensity which depends on the doping concentration and ceramization temperatures. The size (20 nm) of the nanocrystallites was estimated from the Sherrer's formula and found to increase with increasing ceramization temperature, small enough to avoid scattering losses and ensure an excellent transparency of the glass-ceramics comparable with that of the parent glass. An enhancement of the luminescence properties of Yb~(3+) ions surrounded by a crystalline low phonon environment is observed. Finally, the utilization of these heavily Yb~(3+)-doped ultra-transparent materials for laser cooling and solid state laser applications is discussed.