Study On Crystal Growth And Properties Of Dy³⁺-doped Yellow Laser Crystals

Yellow laser plays an important role in the fields of medical treatment and astronomy.However,the yellow lasers used today mostly rely on non-linear optic crystals or frequency mixing technology,which are suffering from the more complicated instruments and lower efficiency.Nowadays,with the development of blue InGaN LD,there is a new way to obtain yellow laser output.Using the InGaN LD as the pump source,the yellow laser can be generated directly from the Dy³⁺-doped laser crystals.It’s reported that the yellow laser output has been successfully realized in Dy³⁺:Y₃Al₅O₁₂,Dy³⁺:LiLuF₄,Dy³⁺/Tb³⁺:LiLuF₄crystals,and so on.However,limited by the small absorption cross-section of Dy³⁺ion in the blue light region and the low pumping efficiency,none of the crystals can meet the demands of applications till now.Since the spectral properties of rare-earth ions can be affected to some extent by the surrounding crystal fields,it is feasible to improve the laser property by adopting new Dy³⁺-doped matrices.In this work,three novel laser crystals of Dy³⁺:Ca₃NbGa₃Si₂O₁₄,Dy³⁺:Ca₃Gd₂（BO₃）₄,and Dy³⁺:Gd₂SrAl₂O₇ are presented.The phase structures,spectral and thermal properties of these three crystals were analyzed to evaluate their potential as yellow laser crystals.A series of Ca₃NbGa₃Si₂O₁₄ polycrystalline powders with different concentrations of Dy³⁺were synthesized.The study of the XRD refinement data of these powders shows that with the increased concentration of Dy³⁺,the lattice parameters of Ca₃NbGa₃Si₂O₁₄polycrystalline powders decrease.That means the Dy³⁺ions have been introduced into the Ca₃NbGa₃Si₂O₁₄ lattice by replacing the Ca²⁺ions with larger ionic radii.The fluorescence properties of the polycrystalline powders were performed to select the optimum concentration for crystal growth.After that,a 5.58 at.%Dy³⁺:Ca₃NbGa₃Si₂O₁₄ single crystal was successfully grown by Czochralski method.The spectral properties and Judd-Ofelt analysis were carried out.The crystal has a relatively strong and wide absorption peak at about 453 nm with an absorption cross-section of 1.3×10^-21 cm² and a full width at half maximum（FWHM）of 10.2 nm.The Judd-Ofelt parametersΩ₂,Ω₄ andΩ₆ were calculated to be 4.51×10^-20 cm²,2.93×10^-20 cm² and 0.19×10^-20 cm²,respectively.The emission cross-section at 572 nm is 1.8×10^-21 cm² with a FWHM of 16.3 nm.The fluorescence time of the ⁴F_9/2→⁶H_13/2 transition is 0.24 ms.The results mean that Dy³⁺:Ca₃NbGa₃Si₂O₁₄crystal is a potential yellow laser crystal.A series of Dy³⁺-doped Ca₃Gd₂（BO₃）₄ polycrystalline powders with different Dy³⁺concentrations were prepared.The refinement of the XRD data shows the lattice parameters shrank gradually with the increasing contents of Dy³⁺ions,which means that the Dy³⁺ions with smaller ionic radius substituted the ions with larger radius in the substrate.According to the results of ICP and the previous research results,it can be concluded that the Gd³⁺ions in the substrate have been replaced.Comparing the fluorescence properties of the Dy³⁺-doped Ca₃Gd₂（BO₃）₄ polycrystalline powders,a proper doping concentration was determined for crystal growth.Then,a 6.36 at.%Dy³⁺:Ca₃Gd₂（BO₃）₄ crystal was successfully grown by Czochralski method.The spectroscopic properties and Judd-Ofelt analyses of the crystal were performed.The Judd-Ofelt parametersΩ₂,Ω₄ andΩ₆ were obtained to be 2.79×10^-20 cm²,2.10×10^-20 cm² and 2.28×10^-20 cm²,respectively.The absorption cross-section at 453 nm is 0.95×10^-21 cm² with a FWHM of 10.2 nm,which is suitable for InGaN LD.The yellow light emission cross-section at 576 nm is 2.18×10^-21cm² with a FWHM of 14.0 nm.The fluorescence lifetime of ⁴F_9/2→⁶H_13/2 transition was measured to be 0.26 ms.These results reveal that the Dy³⁺:Ca₃Gd₂（BO₃）₄ crystal is a promising yellow laser candidate.A proper concentration for crystal growth was screened out by comparing the fluorescence properties of the Dy³⁺-doped Gd₂SrAl₂O₇polycrystalline powders with different concentrations.After then,a 3.43 at.%Dy³⁺:Gd₂SrAl₂O₇ crystal was grown by Czochralski method successfully.The spectral properties,thermal properties and Judd-Ofelt analysis of the crystal were investigated in detail.The Dy³⁺:Gd₂SrAl₂O₇ crystal shows a relatively large absorption cross-section at 453 nm,which is 2.31×10^-21 cm².The Judd-Ofelt parametersΩ₂,Ω₄ andΩ₆ were obtained to be 1.33×10^-20 cm²,2.81×10^-20 cm² and2.79×10^-20 cm²,respectively.The yellow light emission cross-section is 1.81×10^-21 cm²with a FWHM of 16.7 nm.The fluorescence lifetime of ⁴F_9/2→⁶H_13/2 transition was measured to be 0.22 ms.The average thermal expansion coefficients along a-and c-axis are 10.2×10^-6 K^-1 and 15.4×10^-6 K^-1,and the thermal conductivities are 5.0 W/（m·K）and5.2 W/（m·K）,respectively.The crystal shows a relatively small anisotropy in thermal properties.All these results indicate that the Dy³⁺:Gd₂SrAl₂O₇ crystal is a potential yellow laser crystal.