Studies on the growth and properties of stoichiometric lithium tantalate crystals

钽酸锂晶体由于具有优良的压电、铁电、非线性、电光等性能，被广泛地应用于声表面波、电光、声光、集成电子、信息存储等方面，并且其应用领域还在不断扩大。人们对钽酸锂晶体的质量和性能也提出了越来越高的要求。研究表明，提高钽酸锂晶体中的锂含量可以大幅度提升晶体各方面的性能；另外，掺杂改性也是提高钽酸锂晶体性能的重要途径。 本文采用富锂提拉生长出了大尺寸、无宏观缺陷、光学质量较好的纯的近化学计量比钽酸锂晶体（SLT）和一系列不同掺杂浓度的Mg:SLT（Mg的摩尔浓度分别为1at%，2at%和3at%）晶体，并通过多种分析测试方法对晶体的结构和性能进行了研究。最后还尝试生长了双掺Yb和Mg的LiTaO3晶体，并较全面地评估了其光学性能。 本文主要内容包括以下几个方面： （1）简要概述了LiTaO3晶体的研究进展、生长方法以及Li含量的表征方法。 （2）详细叙述了富锂提拉法生长SLT晶体的工艺，通过选择合适的原料配比和工艺参数，成功生长出了SLT，1at%Mg:SLT，2at%Mg:SLT，3at%Mg:SLT晶体，晶体尺寸约为φ50×50mm。 （3）X-射线粉末衍射实验结果表明富锂提拉法生长的SLT晶体结晶质量非常好， Mg的掺杂并没有改变LiTaO3晶体的结构，只是使晶胞参数有所增大。 （4）测试了SLT和Mg:SLT晶体的紫外吸收边、红外吸收谱和显微拉曼光谱。结果表明，与SLT晶体相比，Mg:SLT晶体的紫外吸收而发生了红移，而OH-振动红外吸收峰明显地向高能端移动，这说明Mg的掺杂浓度已经达到了晶体的抗光损伤阈值浓度。根据锂空位模型，分析了Mg2+在LiTaO3晶体中的占位情况。还利用中Li含量与紫外吸收边和拉曼线宽的关系，计算了晶体中的Li含量，结果表明样品中的Li含量都非常接近化学计量比，随着Mg浓度的增加，晶体中的Li含量随之下降。 （5）通过电滞回线测试了SLT和Mg:SLT晶体在室温下的矫顽场，利用观察光斑畸变的方法测试了晶体的抗光损伤性能，结果表明Mg的掺杂能有效地降低LiTaO3晶体的矫顽场并大幅提高LiTaO3晶体的抗光损伤阈值。 （6）尝试生长了1at%Yb,2at%Mg:SLT和1at%Yb,5at%Mg:CLT晶体，通过对Yb3+的吸收光谱、荧光光谱和荧光寿命的测试分析以及光谱参数的计算，研究了Yb3+离子在LiTaO3晶体中的光学性能，并评估了LiTaO3晶体作为激光基质以及在激光自倍频方面的应用前景。Due to its excellent piezoelectric, ferroelectric, electro-optic, and nonlinear optical properties, Lithium tantalate crystal is wildly used in surface acoustic wave, electro-optic, acousto-optic, integrated electronic and holographic storage，and its applications are still expanding. People also have higher and higher requirements on lithium tantalite crystal’s quality and performance. Previous studies show that improving the lithium content in LiTaO3 crystal can significantly improve its performance in various aspects; In addition, doped-LiTaO3 is another important way to improve the crystal’s properties. In this paper, stoichiometric lithium tantalite (SLT) and Mg-doped SLT crystals (Mg concentration were 1at%, 2at% and 3at%) with large size, no macro-defects and high optical quality were successfully grown by the Li-rich Czochralski method. Their structures and properties were studied by a variety of methods. Besids, we tried to growth Yb and Mg co-doped LiTaO3 crystals and made a comprehensive estimation on their optical properties. The research work is summarized as the following: (1) Research progress of LT crystals, growth techniques of SLT crystals and methods of determining Li2O content in LT crystals were reviewed. (2) The growth procedure of SLT crystal was introduced in detail. By choosing appropriate material ratio and growth parameters, SLT and Mg:SLT crystals were successfully grown from the Li-rich melt. The crystals’ sizes were about φ50×50mm, with on macro-defects observed. (3) XRD results indicated a very good crystallization of the SLT crystals grown by the Li-rich Czochralski method. Exist of Mg in the crystals resulted a little increase in lattice parameters, but did not chage the crystal structure. (4) UV absorption edge, infrared absorption spectra and micro-Raman spectra of SLT and Mg:SLT crystals were measured. In cases of Mg:SLT crystals, the UV absorption edges showed red-shift and the infrared absorption bands moved towards high-energy side compared with those of SLT, which means that the concentration of Mg in the crystals has exceeded the threshold concentration. On the basis of Li-vacant model, we discussed the Mg2+ locations in LT crystals. By using the relation between Li-content and UV absorption edge and FHW, the Li conctents in crystals were calculated and they decreased with increase of Mg concentration. (5) The coercive field and photodamage resistance thresholds of the crystals were obtained by testing the electic hysteresis loops and observing laser spot distortion. The results suggested that introducing of Mg into the LT crystal can effectively reduce LT crystal’s coercive field and improve its photodamage resistance substantially. (6) We tried growing 1at%Yb,2at%Mg:SLT and 1at%Yb,5at%Mg:CLT crystals. By testing the absorption spectra, fluoeresence spectrum and lifetime, we calculated the optial parameters of Yb3+ and ananlyzed its optical properties, based on which we assessed the Yb,Mg:LiTaO3 crystal’s potential applications in laser host and devices