Synthesis And Nonlinear Optical Properties Of Ultraviolet Alkali-metal Phosphate Crystal Materials

Ultraviolet（UV）laser source has important applications in microprocessing,UV medical treatment,photoetching,and other fields.The solid laser is a very important part of UV laser source.As the core device of solid laser,UV nonlinear optical（NLO）crystal material has been widely concerned and studied.Because of its rich crystal structure and excellent optical properties,phosphate has become an important system for exploring UV NLO crystal materials.It is found that the birefringences of UV phosphates and the second harmonic generation（SHG）responses are generally small.To enhance the birefringence and SHG response,a variety of new UV phosphate nonlinear optical materials were successfully synthesized by combining different atoms with alkali metal phosphate.And their related properties were characterized.At the same time,the structure of these compounds and the structure-property relationship are studied in detail.A new phosphate compound,α-Li₂Na₂P₂O₇ was successfully synthesized through the high temperature solution method.The SHG response ofα-Li₂Na₂P₂O₇is 0.5 times that of KDP.First-principle calculations indicate that the SHG response ofα-Li₂Na₂P₂O₇ mainly comes from the P-O group,and its birefringence is0.008@1064 nm.We introduce the Al atom into the alkali-metal phosphate system and synthesize A₃Al₂（PO₄）₃（A=Rb,K）compounds by the high temperature solution method.The birefringences of Rb₃Al₂（PO₄）₃ and K₃Al₂（PO₄）₃ are 0.0072 and0.0075@1064 nm,respectively,and their SHG responses are 0.6 and 0.4 times that of KDP,respectively.First-principle calculations indicate that their SHG response mainly comes from the P-O group,while the contribution of the Al-O group is quite small.This means the introduction of the Al-O group does not enhance the birefringence and SHG response of the materials.To enhance the birefringence of UV phosphates,a new UV phosphate nonlinear optical material K₂TeP₂O₈ was obtained by introducing the Te atom into the alkali-metal phosphate system through the high temperature solution method.The diffuse reflectance spectrum indicates that the band gap of K₂TeP₂O₈ is 4.6 e V.First-principle calculations indicate that K₂TeP₂O₈ has a large birefringence（>0.05@400-1400 nm）among the UV phosphates.Considering the coplanar arrangement of the Te-O group in the structure,it is indicated that introducing Te into the alkali-metal phosphate with a coplanar arrangement of the Te-O group is a useful way to enhance the birefringence.To enhance the SHG responses of UV phosphates,we introduce the Pb,and Bi atoms with lone pair electrons into the alkali-metal phosphate system and obtain RbPbBi₂（PO₄）₃ and Li₂ABi（PO₄）₂（A=K,Rb,Cs）.The SHG response of RbPbBi₂（PO₄）₃ is 0.7×KDP.The calculated birefringences of Li₂KBi（PO₄）₂,Li₂Rb Bi（PO₄）_2,and Li₂Cs Bi（PO₄）₂ are 0.024,0.035 and 0.024@1064 nm,respectively,which are relatively large among UV phosphate materials.Li₂KBi（PO₄）₂,Li₂Rb Bi（PO₄）_2,and Li₂Cs Bi（PO₄）₂ have large SHG responses,which are 5.0,3.1,and 2.50 times of KDP,respectively.Among all bismuth phosphates,Li₂KBi（PO₄）₂ shows the largest SHG response.It is indicated that introducing the Bi atom into the alkali-metal phosphate system may be a useful way to enhance the SHG response and obtain relatively large birefringence.Through the study of the influence of alkali-metal cations on the SHG response,we find out that the alkali-metal cations can influence the arrangements of units,which further influence the SHG response.We have studied the influence of the arrangement of PO₄ tetrahedra in the P₂O₇ group on the SHG response.It is indicated that the uniform arrangement of PO₄ tetrahedra in the P₂O₇ group is helpful to enhance the SHG response of diphosphates.We find phosphates with isolated Cs-O、Rb-O units will have a high probability to possess disordered structures.We use isolated soft units to explain this phenomenon.We have successfully synthesized a series of UV phosphate NLO materials.Among them,K₂TeP₂O₈ have large birefringence and Li₂ABi（PO₄）₂（A=K,Rb,Cs）exhibit large SHG responses.These results are useful and helpful for the design and exploration of UV phosphate NLO materials.