Rare Earth Doped Transparent Phosphate Glass Ceramics Preparation And Photoluminescence

In recent years, Er³⁺ and Er³⁺/Yb³⁺ co-doped phosphate glasses have attracted much attention as host materials for microchip lasers and fiber lasers and amplifiers operating at the eye-safe wavelengths of 1.54μm. They have wide applications including optical communication, range finding, coherent optical transmission and etc. The co-doped glasses exhibit high solubility of rare earth ions, large stimulated emission cross-section, long radiative lifetime, high energy transfer efficiency from Yb³⁺ to Er³⁺, and weak interaction among active ions.Unfortunately, their relatively poor thermal and chemical durability prevents them from optical properties and widely practical applications. Recently, rare earth (RE) doped transparent glass ceramics (TGC) have been investigated as laser materials since they exhibit high luminescent efficiency, Vis-NIR transparency, chemical stability, and so on. Such glass ceramics are characterized with their adjustable structure, controllable phase crystallizing and heavy RE³⁺-doping level. More importantly, the glass ceramics have better properties of thermal conductivity and thermal shock, which make them more suitable to be laser materials for high-power devices. In order to combine the good performance of the erbium doped phosphate glass and the nanostructure of glass ceramic, we fabricated Er³⁺/Yb³⁺ co-doped phosphate glass ceramics by the high-temperature melting technique. Using different thermal analysis (DTA), x-ray diffraction (XRD), transmittance electronic microscope (TEM) or scanning electronic microscope (SEM), photoluminescence spectra (PL), and Judd-Ofelt (J-O) theory, McCumber theory, and Fuchtbauer-Ladenburg (F-L) formula, we studied the correlation among heat treating techniques, precipitated crystal size, crystallinity, transmittance and luminescence behaviors. A series of important conclusions and innovative results with practical significance were obtained, which provide the foundation for further development and application of RE ions doped phosphate glass ceramics.Transparent 37P₂O₅-31.4CaO-25.6Na₂O-6Al₂O₃-0.25Er₂O₃-7.5Yb₂O₃ (mol %) phosphate glass ceramics have been prepared. The spectroscopic properties of Er³⁺ in the glass and glass ceramics have been studied. XRD results indicated that the nanocrystals were YbPO₄ and ErPO₄ and the crystal size and crystallinity increased with the heat treating time increasing. The upconversion and near infrared emissions of the Er³⁺ ions in the glass ceramics are increased significantly with the precursor glass. This agrees well with the varation of the crystal size and crystallinity. Calculation using J-O theory, McCumber theory and F-L formula confirmed that Er³⁺ ions entering into the nanocrystals led to their higher ligand symmetry and weaker covalence properties than in the glass matrix, which madeΩ₂ decreased and further improved the properties of upconversion luminescence and 1.54μm near infrared emission efficiency, effective width, and gain parameters. The effect of temperature on the luminescence intensity of up-conversion and near infrared in Er³⁺/Yb³⁺ co-doped phosphate glass ceramics has been investigated. The fluorescence intensity is changing at different temperature and the results are explained with the level transitions in Er³⁺/Yb³⁺ co-doped system. Meanwhile, the lifetime of Er³⁺:⁴I_13/2 level corresponding to different operating temperature and pump power is also discussed, and the experimental results are fitted using multiphonon relaxation theory. These results will be helpful to further optimize parameters of lasers materials and high gain fiber devices.We prepared Ce³⁺/Er³⁺/Yb³⁺ tri-doped transparent phosphate glass ceramics and discussed the influence of energy acceptors Ce³⁺ ions on the upconversion and 1.5μm emission properties of Er³⁺ in the glass ceramics. The phonon-assisted energy transfer between Er³⁺ and Ce³⁺ (Er³⁺:⁴I_11/2+Ce³⁺:²F_5/2→Er³⁺:⁴I_13/2+Ce³⁺:²F_7/2) accelerated population feeding from the ⁴I_11/2 to the ⁴I_13/2 level, and therefore drastically decreased the upconversion emission of Er³⁺ under 975 nm LD excitation. Meanwhile, the near infrared luminescence enhanced greatly with the introduction of Ce³⁺ ions at the proper concentration. The Ce³⁺/Er³⁺/Yb³⁺ tri-doped phosphate glass ceramics will be preferable for efficient 980 nm pumped Er³⁺-doped optical amplifiers and lasers.Luminescence TGCs containing LiPO₃ and TiP₂O₇ have been prepared successfully. We investigated the effects of heat treatment conditions on the crystal size, Vis-NIR transparency, the upconversion and near infrared luminescence of Er³⁺ ions in the glass ceramics. The upconversion luminescence intensity of Er³⁺ ions in the glass ceramics increased significantly with increasing heat treating temperature or time under 975nm wavelength excitation. Stark split near infrared emission peaks of Er³⁺ have been observed in the glass ceramics, and the effective bandwidth increases with increasing heat treating temperature. PL spectra measurements and J-O calculation reveal that Er³⁺/Yb³⁺ co-doped TGCs have much broader 1.5μm bandwidth, much stronger 1.5um luminescence, red and green upconversion luminescence than the precursor glass. For the co-doped TGCs, both of their 1.5μm quality factors for bandwidth (σ_e×Δλ_eff) and for gain (σ_e×τ_mea) are better than ZBLAN. For the glass ceramic heat treated at 480℃for 4h, the bandwidth and gain are 1.33 and 1.22 times than that of ZBLAN, respectively.