氟硼酸盐短波长非线性光学晶体的结构性能关系及合成研究

Nonlinear optical (NLO) crystals are the key materials to extend the working wavelength range of solid-state lasers to ultraviolet/deep-ultraviolet (UV/DUV) region and be increasingly used in industry, scientific research, and military. At present, the research difficulty of DUV NLO materials is obtaining large-size crystals which meet the performance requirements of "Deep-ultraviolet transmittance-Large effective SHG coefficients-Appropriate birefringence". To overcome this difficulty, the applicant proposed a material design strategy. Introducing fluorooxoborates groups (including (BO3F)4-, (BO2F2)3- and (BOF3)2-) in borates to balance the above criteria. Theoretical and experimental studies have shown that the introduction of fluorooxoborates groups could break through the fixed 3D B–O network that would produce a larger birefringence without layering and simultaneously keep a short cutoff edge down to DUV. It is necessary to carry out a detailed study on how the luorooxoborates groups affect crystal structure and the corresponding NLO properties of fluorooxoborates. In this project, the influence mechanism of the introduction of fluorooxoborates groups on the material topological structure, birefringence, band gap and SHG effect will be studied based on the AIM (Atoms in Molecules) real space atom cutting method developed by the applicant and accordingly to design and synthesis new fluorooxoborates. It will provides a new strategy for the discovery of DUV NLO materials.

非线性光学晶体是拓展固体激光器波长至紫外/深紫外波段的关键材料，在工业、科研及军工领域有重要的应用。目前深紫外非线性光学材料的研究难点在于获得同时满足“深紫外透过-大的有效倍频系数-合适的双折射”性能要求的大尺寸晶体。针对这一挑战，申请人经过前期研究，提出了在硼酸盐体系中引入氟硼酸基元(包括(BO3F)4-、(BO2F2)3-和(BOF3)2-)来平衡上述各项性能要求的材料设计策略。理论和实验研究发现，引入氟硼酸基元可以产生大的倍频效应和双折射，同时有利于深紫外透过。现阶段需要开展氟硼酸基元对晶体结构以及非线性光学性能调控的详细研究。本项目基于申请人自主研发的AIM (Atoms in Molecules)理论原子实空间切割软件等方法，研究氟硼酸基元的引入对材料拓扑结构，双折射，带隙和倍频效应的影响机制，设计合成新的氟硼酸盐材料。为深紫外非线性光学材料的设计合成提供一种新策略。

围绕本项目的预期目标，我们开展了针对性和创新性的工作。（1）围绕氟硼酸基元对晶体结构以及非线性光学性能调控的影响，对系列氟硼酸盐开展了结构性能关系研究。利用遗传算法晶体结构预测方法，获得了系列新晶体构型。这些结构丰富了(BO3F)4-、(BO2F2)3-基团与(BO3)3-基团的连接方式，为研究各功能基团对材料带隙、双折射、倍频效应等性能的影响提供了素材。从而揭示了氟硼酸基元在晶体结构中对材料性能的影响，为设计性能优异的非线性光学材料提供了线索。（2）发现了新型氟硼酸酸盐非线性光学晶体，初步性能评估表明，该晶体可实现深紫外区相位匹配，具有大的倍效应，宽的紫外透过范围和合适的双折射。（3）将研究方向拓展到氟磷酸盐体系，提出用 (PO3F)2−和(PO2F2)–基团替换(PO4)3−基团克服磷酸盐材料双折射过小无法实现深紫外相匹配的材料研发策略，筛选并生长出非线性光学材料(NH4)2PO3F。（4）初步发展了第一性原理非线性光学晶体材料高通量筛选管线系统，将其应用于硼酸盐体系，筛选出若干性能优异的紫外/深紫外非线性光学晶体。紧接着，将该系统应用于锗酸盐体系，获得了若干有望因为于3-5微米大气透明窗口的中红外非线光学材料。（5）在前面研究的启发下，继续寻找含有混合阴离子基团的非线光学材料，发现并提出了氮硅酸盐可以克服硅酸盐双折射过小的不足，是一类潜在的紫外非线性光学材料。