Design,Synthesis And Properties Of Alkali-metal Fluoroaluminoborate Nonlinear Optical Crystal Materials

Using the high-power all-solid-state laser as the fundamental frequency light source,the deep ultraviolet coherent light generated by the multi-stage frequency conversion technology based on nonlinear optical crystals has the advantages of high efficiency,simple and compact structure,good output beam quality,and long life.It has important applications in laser lithography technology,laser precision processing,photoelectron spectrometer,laser spectrometer,laser Raman spectrometer,and other high-tech and advanced instrument manufacturing fields.The key to realizing deep ultraviolet laser output lies in suitable nonlinear optical crystal materials.However,there are few nonlinear optical crystal materials which can be used in the deep ultraviolet range with excellent comprehensive performance.This thesis is devoted to exploring new types of deep ultraviolet nonlinear optical crystal materials.Taking the microstructures as the starting point,through the design and selection of the structural units,a series of fluoroaluminoborates containing both[B₃O₆]and[Al O₃F]groups were obtained.Compared with other similar materials,the[B₃O₆]group with?-conjugation effect is beneficial to improve the frequency doubling effect and optical anisotropy of the material,while the[Al O₃F]group improves the thermal stability of the crystal.At the same time,the alkali metal cations without d-d or f-f transition are chosen,which is beneficial to the blue shift of the absorption edge and to obtain a wide transmission range.In addition,compared with the classic KBBF crystal material,while retaining the excellent performance,the as-synthesied fluoroaluminoborates do not use high toxic raw materials Be O,which is beneficial to the growth of large-size crystals.The specific research contents are as follows:(1)The alkali metal fluoroaluminoborate CsAlB₃O₆F was synthesized for the first time.This compound crystallizes in the orthorhombic system,Pna2₁(No.33)space group.The unit cell parameters are a=8.064(3)?,b=11.970(4)?,and c=7.036(3)?.The powder polycrystalline samples of CsAlB₃O₆F were synthesized by high temperature solid phase method.Through the infrared spectroscopy and elemental analysis of CsAlB₃O₆F,the rationality of its structure is proved;Thermal performance test shows that it is a non-congruent melting compound;The result of ultraviolet-visible diffuse reflectance spectroscopy measurement indicates that its ultraviolet cut-off edge is less than 190 nm;In addition,CsAlB₃O₆F has a large second harmonic generation effect(2×KDP)according to the Kurtz-Perry method.Moreover,based on the density functional theory,theoretical calculations show that CsAlB₃O₆F has a short UV cut-off edge(166 nm,band gap 7.49 e V),large birefringence(0.091@1064 nm),and a short phase matching wavelength(182 nm).The second harmonic generation charge density map shows that the main contribution of the nonlinear optical effect comes from the[B₃O₆]groups.Further,the flux method was used to explore the crystal growth process,and the CsAlB₃O₆F crystal was initially obtained in the open system.(2)The mixed alkali metal fluoroaluminoborate Cs_0.5Rb_0.5Al B₃O₆F was obtained for the first time by the high-temperature flux method.Cs_0.5Rb_0.5Al B₃O₆F crystallizes in the hexagonal system,P6?2c space group,and its unit cell parameters are a=b=6.9807(10)?,and c=8.057(3)?.The basic structural units of Cs_0.5Rb_0.5Al B₃O₆F are the[B₃O₆]ring and[Al O₃F]tetrahedron,and these units are connected with each other forming a two-dimensional[Al B₃O₆F]_?layer extending along the bc plane,in which eighteen-membered rings are observed.The Cs and Rb cations are filled between two adjcent[Al B₃O₆F]_?layers.The thermal analysis,ultraviolet-visible diffuse reflectance spectrum,powder second harmonic generation measurement were performed.The results show that the UV cut-off edge of Cs_0.5Rb_0.5Al B₃O₆F is less than 200 nm,and its second harmonic generation effect is 2×KDP.On this basis,Cs_xRb_1-xAl B₃O₆F(0.2<x<0.8)solid solution compounds with different ratios of Cs and Rb were obtained.(3)The alkali metal fluoroaluminoborate Rb Al B₃O₆F was obtained for the first time.The crystal structural analysis indicates that Rb Al B₃O₆F crystallizes in the orthorhombic system Pna2₁(NO.33)space group,and the unit cell parameters are a=22.403(4)?,b=12.311(2)?,and c=7.2051(13)?.The infrared spectrum,ultraviolet-visible diffuse reflectance spectrum,element analysis,thermal analysis and theoretical calculations were performed.The results show that the cut-off edge of Rb Al B₃O₆F is less than 200 nm,and the powder second harmonic generation effect is 1/5×KDP.Theoretical calculation results show that its band gap value is 7.15 e V(based on HSE06 hybridization functional),and the birefringence is0.0946@1064 nm.In addition,the shortest phase matching wavelength of Rb Al B₃O₆F is down to 174 nm.Through the study of alkali metal fluoroaluminoborates,we found that as-obtained compounds have good thermal stability,and they are easy to obtain single crystals in an open system.They also have good linear and nonlinear optical properties(wide transparency range,large nonlinear optical effect,and suitable birefringence to achieve phase matching in deep ultraviolet range),and the nonlinear optical effects can be adjusted by controlling the radius of the metal cations.Therefore,the alkali metal fluoroaluminoborate is the optimal system for searching potential deep-ultraviolet crystal materials.The research results obtained in this paper can also provide reference for the future research in fluoroaluminoborate system.