Yb-doped Ion Of Transparent Glass-ceramic Materials

The laser materials doped Yb³⁺ ions have become the hotspot because Yb³⁺ ion has very simple energy level system, good energy storage ability, long fluorescence lifetime, no excited state absorption, and coupling effectively with the wavelength (900-1100nm) pumped by solid diode laser.Transparent oxyfluoride glass ceramics doped Yb³⁺ ions have attracted widespread interest due to their excellent optical properties like fluoride nanocrystals and good mechanical, chemical properties like oxide glasses. Compared with other fluoride , CaF₂ has high solid solubility to rare earth, wide transmission range (0.125-10um) ,good chemical stability , and lower phonon energy, so the transparent oxyfluoride glass-ceramics containing CaF₂ doped with Yb³⁺ become very potential laser material, especially in the field of tunable laser and ultra short pulse laser.In this paper, the transparent oxyfluoride glass-ceramics of NaF-CaF₂ -Al₂O₃-SiO₂ system doped Yb³⁺ ions were prepared by conventional melt-quenching method and optimized heat treating regime. The glass-forming ability, crystallization mechanism, microstructure of NaF-CaF₂ -Al₂O₃-SiO₂ system glasses were analyzed by differential thermal analysis (DSC), X-ray diffraction (XRD) analysis, scanning electronic microscopy (SEM) and transmission electron microscopy (TEM). The absorption spectrum, fluorescent spectrum and fluorescent lifetime decay curve of the glass-ceramics samples were measured. The influences of Yb³⁺ content on the spectrum properties of glass-ceramics were discussed. The results show that the glass-forming ability of NaF-CaF₂ -Al₂O₃-SiO₂ system glasses are weakened as introducing alkali metals oxides.The reason is that the addition of alkali metals oxides increases the ratio of O:Si in the glass structure and decreases the viscosity of the glass melt.As a result, the ions are easier to migrate and adjust into lattice structure during molding process. When CaF₂ is replaced by CaO, the Si-O network of glass is strengthened.the glass forming ability is improved. Obviously, a large amount of network disconnected points or non-bridge oxygen ions exist in the network structure formed by connecting [SiO₄] and [SiO₄] or [SiO₄] and [AlO₄] simce the molar percentage of NaF and CaF₂ are higher than 30% in NaF-CaF₂-Al₂O₃-SiO₂ system glass. The Ca²⁺ and Na⁺ ions enter into the intermittent points and bond with non-bridge oxygen ions (such as≡Si-O^-) in the network structure. When introducing Yb₂O₃ into the glass, Yb³⁺, Ca²⁺ and Na⁺ ions co-exist in the locations of network discontinuously. Due to its higher valence, Yb³⁺ ions have the priority to bond the non-bridge oxygen ions, which results in the concentration increasing of Ca²⁺ and Na⁺ ions located in the discontinuities. Part of the Ca²⁺ and Na⁺ ions that have not yet reached valence saturation will bond with the non-bridge oxygen ions, most of which will connect the F^- orfree oxygen ion O^2-. Compared with the Ca²⁺ ions bonding to non-bridge oxygen ions, the Ca²⁺ ions bonding to the F^- ions are more prone to migrate, rotate, rearrange and adjust into lattice structure during heat treatment process. With the increasing of Yb₂O₃ content, the number of Ca²⁺ ions bonding to non-bridge oxygen ions decrease and the number of the Ca²⁺ ions bonding to F^- ions increase. Therefore, the crystallization activation energy declines gradually with Yb₂O₃ content, resulting in the increasing of crystallization ability for this system glass. During the crystallization, the crystal grains grow to three-dimension directions when the basic glasses are treated at the crystallization temperature, and the crystallization is controlled by diffusion. The increasing of the crystallization leads to the reduce of nucleation rate, ultimately to zero. In the glass-ceramics of NaF-CaF₂ -Al₂O₃-SiO₂ system, the positions of two emission peaks both shift toward long wavelength and the ratio (I_λS/I_λp) of relative intensity between the second peak (I_λS) and that at the first peak (I_λp) in the fluorescence spectrum rises with increasing of Yb³⁺ content. During the heat treatment, Yb³⁺ ions are prior to incorporate into CaF₂ nano-crystals and form various asymmetric crystal lattices, so the energy level structure of Yb³⁺ becomes more complex and the amount of electrons excited from ground state increases, resulting in an enhanceing absorption optical density/ emission intensity and a broadening absorption spectrumand and fluorescent spectrum. With Yb³⁺ content increasing, fluorescence lifetime rises initially and then descends, which is attributed to the effects of radiative trapping and fluorescence quenching. Both the absorption cross-section and the stimulated emission cross-section of the glass ceramics decrease monotonously because of the concentration quenching effect of Yb³⁺. The prepared transparent glass ceramics has many excellent properties, such as a higher absorption cross-section (2.68pm²), a bigger stimulated emission cross-section (5.51pm²), a longer fluorescence lifetime (1.32ms) saturation pump intensity (5.77Kw/cm²) and a minimum pump dynamics (0.60 Kw/cm²).