Crystallization And Mid-infrared Luminescence Properties Of Er³⁺-doped Oxyfluoride Glass-ceramics

Laser operating at³μm wavelength region has attracted considerable attention recently due to its potential applications in laser radar,remote sensing,medicine,and pump source for mid-infrared lasers.Currently,the main laser host material for rare earth doped solid state lasers is fluoride glass and fibers,especially ZrF₄-BaF₂-LaF₃-AlF₃-NaF（ZBLAN）system,however,fluoride glasses have poor mechanical properties,low damage threshold and complex manufacture process.It is necessary and meaningful to develop new kind of mid-infrared host materials to improve the³μm laser.Transparent oxyfluoride glass ceramics have potential applications in this area as they combine the advantages of oxide glasses and fluoride crystals which give them good mechanical and luminescent properties.In the present work,a series of compositions were studied to fabricate transparent glass ceramics,and much attention were paid on the 2.7μm emission from Er³⁺in glass ceramics.This work would lay the foundation of novel host material for³μm laser.This dissertation is composed of five chapters.In chapter 1,the characters and applications of³μm laser,and the active ions and mechanisms to generate fluorescence and laser at³μm were introduced.The research progress in³μm rare earth doped solid state lasers and oxyfluoride glass ceramics were reviewed.In chapter 2,the sample preparation procedures and physical and spectroscopic properties measurements were introduced.The related spectroscopic parameters calculation theories were also introduced briefly.In chapter 3,the crystallization and luminescence properties of Er³⁺doped glass ceramics containing NaYF₄ nanocrystals were studied.NaYF₄ nanocrystals were successfully obtained in silicaluminate glass matrix.The segregation of Er³⁺ions in nanocrystals was observed by TEM measurement,indicating that Er³⁺ions preferentially incorporate into nanocrystals.The calculatedΩ₂ values in J-O intensity parameter did not decrease in glass ceramics comparing to precursor glass,which is different to previous reports,indicating the residing of Er³⁺ions in high symmetry and low covalent bond sites in precursor glass.Upon 980 nm laser excitation,intense 2.7μm emission was obtained in glass ceramics and the intensity increased with the raise of heat treatment temperature.The maximum emission cross section was about0.64×10^-20 cm²,and positive gain could be achieved in the whole 2.7μm band when p≥0.6 in the gain cross section,the result indicates the potential application of this material as mid-infrared host material.In order to adjust the properties of glass ceramics,effect of adjusting Na₂O/NaF ratio on the crystallization and luminescence properties of glass ceramics was also studied.Transparent glass ceramics could also be obtained despite of the change in glass composition,XRD and TEM measurement evidenced the formation of cubic NaYF₄ crystal in all systems,but the XRD peak positions shift towards higher 2θdirection with the raise of Na₂O content,indicating that the crystal structure were distorted.2.7μm emissions were achieved in all sample with different composition,and the Stark split in emission spectra got more obvious with the raise of Na₂O content while the emission intensity decrease.In chapter 4,the effect of adjusting composition on the crystallization and luminescence properties in glass ceramics containing lutetium oxyfluoride nanocrystals were studied.Transparent glass ceramics were obtained in all glass sample with different Na₂O/NaF ratio.Particularly,orthorhombic Lu_nO_n-1F_n+2（n=5-10）nanocrystals were crystallized in glass with Na₂O/NaF equals 12/18 and 15/15,while cubic LuOF nanocrystals were obtained in glass with Na₂O/NaF equals 18/12.Glass ceramics containing Lu_nO_n-1F_n+2（n=5-10）nanocrystals were found possessing higher 2.7μm emission intensity when excited by 980 nm laser diode.The glass with Na₂O/NaF equals 15/15 was studied thoroughly due to its better comprehensive properties.The calculatedΩ₂ value in J-O intensity parameter was found increasing in glass ceramics comparing to precursor glass,and the spontaneous radiative transition probabilities and branching ratios of Er³⁺:2.7μm emission transition increased slightly.The 2.7μm emission intensity and the lifetime value of its upper transition energy level increased with the raise of heat treatment temperature,and the maximum emission cross section was about 0.52×10^-20 cm²,and positive gain could be achieved in the whole 2.7μm band when p≥0.6 in the gain cross section.In order to adjust the emission property in glass ceramics and fabricate glass ceramics through designing,the effect of substitution of SrF₂ by RF₃（R=Y,La,and Gd）in glass ceramics containing SrF₂ nanocrystals was studied in chapter 5.The slight substitution by RF₃ in glass composition improved the crystallization of glass ceramics and the incorporation of Er³⁺into nancorystals.However,the crystallization decreased when SrF₂ was substituted by large amount of RF₃.With the substitution of SrF₂ by RF₃,the crystalline phase in glass changed from SrF₂ to Sr_1-xR_xF_2+x solid solutions and then Sr₂RF₇ phase.The intensity difference of hypersensive transitions for Er³⁺in glass ceramics and precursor glass was reduced with the substitution by RF₃,which means the reduction of the difference inΩ₂ values between them.Obvious change was found in the split in 1.5μm emission band for glass ceramics after substitution by RF₃,and remarkable enhancement of 2.7μm emission intensity was obtained in glass ceramics with slight introduction of RF₃.