硼酸锂钠深紫外双折射晶体高温熔液法生长及构效关系研究

With the rapid development of deep ultraviolet (< 200 nm) laser technology, the development of deep ultraviolet birefringent crystals used for the preparation of polarizer and optical isolator has lagged behind. Different from the traditional BO3- or B3O6-containing borate research system for deep ultraviolet birefringent crystal, the first alkali metal borate Li2Na2B2O5 with coplanar B2O5 group was invented by applicant, test shows that Li2Na2B2O5 has a large birefringence (0.115@589.3), low ultraviolet cutoff edge (170nm), which is comparable to commercial α-BaB2O4. The researches of crystal growth via high temperature solution method will be performed in this project, including investigating the Li2O-Na2O-B2O3 phase diagram, exploring the flux, solving the problem of coexistence of target phase and heterozygous phase in the process of crystal growth, preparing high quality crystal (> 20×15×10 mm3), characterizing the physicochemical properties, designing and processing the crystal prism, evaluating the application prospect of the crystal in the field of birefringence; revealing the B2O5 units assembly rules and mechanism of the influence of configuration on the birefringence based on the analysis of structural database; promoting the development of borate birefringent materials in deep ultraviolet, and providing a new deep ultraviolet birefringent crystal for international competition.

随着深紫外（< 200 nm）激光技术快速发展，制备分光元件偏振器、光隔离器等的深紫外双折射晶体发展明显滞后。与传统的含BO3或B3O6基元深紫外双折射晶体研究体系不同，前期申请人发明了首例含共平面B2O5基元构型碱金属硼酸盐Li2Na2B2O5，测试表明其具有大的双折射率（0.115@589.3），低的紫外截止边（170 nm），性能媲美商业化α-BaB2O4。本项目将采用高温熔液法，研究Li2O-Na2O-B2O3三元相图，探索助熔剂，解决晶体生长过程中目标相与杂相共存的问题，获得晶体稳定生长工艺并制备高光学质量单晶（> 20×15×10 mm3），系统表征物化性能；确定晶体加工工艺及方案，设计加工棱镜器件，综合评估该晶体在双折射领域的应用前景；基于结构数据库，揭示B2O5基元组装规律及构型对材料双折射率影响机理；推进硼酸盐深紫外双折射材料发展，并提供新的深紫外双折射晶体参与国际竞争。

随着深紫外非线性光学晶体及激光技术的发展，研制能够应用于深紫外波段的双折射晶体成为当前研究热点。与传统的含BO3或B3O6基元深紫外双折射晶体研究体系不同，前期申请人发明了首例含共平面B2O5基元构型碱金属硼酸盐Li2Na2B2O5，本项目针对该晶体开展晶体生长、性能评估及构效关系研究。研究了Li2Na2B2O5晶体熔融特性，根据该晶体非同成分熔融特性，确定了晶体高温熔液法生长方案。确定了以Li2CO3: Na2CO3: H3BO3 = 1.5: 0.5: 2.0的晶体生长比例。优化了晶体生长炉，确定了晶体生长工艺为：在650 oC保持24 h确保熔液熔融澄清均匀，随后单晶炉温度降至546 oC并浸入籽晶。晶体生长过程中单晶炉降温速率为0.2 oC/day；晶转速率为10rpm，待晶体生长到所需尺寸后，将晶体提离液面获得晶体。目前获得了Li2Na2B2O5晶体尺寸有35 × 30 × 11 mm3、35 × 15 × 5 mm3等尺寸晶体。此外，基于B2O5双折射基元进一步设计出性能优异的双折射晶体Li0.5Na0.5AlB2O4F2，并在晶体生长助熔剂探索过程中，获得了具有特殊结构构型或性能的Li4Na2CsB7O14、Na3Al2B7O15等晶体，丰富了硼酸盐晶体结构化学。加工Li2Na2B2O5晶体器件并测试得到晶体在0.187-3.5μm（>60%）范围内的高透明度，深紫外透过截止边为181nm。激光损伤阈值（1064 nm，5 ns）为∼7.5Gw/cm2。热膨胀系数分别为α100 = 47.72 × 10−6·K−1, α010 = 8.54 × 10−6·K−1, α001 = 9.01 × 10−6·K−1。基于棱镜耦合仪对晶体的在可见-红外波段的5个波长折射率进行测试，并拟合Sellmeier方程。Li2Na2B2O5的双折射率较大(n001 − n010 = 0.095@532)。测试表明：Li2Na2B2O5晶体在深紫外双折射领域具有潜在的应用前景。