抗光损伤铌酸锂晶体与体光生伏打效应的研究

Electro-optic Q-switcher is a key device in laser with huge light intensity and finds wide usage in many fields. Lithium niobate Q-switcher is easy for machining, needs no seal, has high transmittance with antireflection coating and a wide working temperature range, so has broad application prospects. But the low resistance to optical damage hinders its practical applications in high light intensity. According to our former research results, we point out at first that the optical damage of lithium niobate is not only depended on photorefraction but also depended on the photorefractive process, which is photorefraction induced by bulk photovoltaic effect will give much heavy optical damage. Therefore, the key method of increasing the resistance against optical damage of lithium niobate is to suppress its bulk photovoltaic effect. However, the physical mechanism of bulk photovoltaic effect in ferroelectrics is still not clear. In this project, we plan to study the physical mechanism of bulk photovoltaic effect in lithium niobate crystals, try to find effective methods to control the bulk photovoltaic effect, then to greatly increase the optical damage resistance of lithium niobate crystals. On the other hand, we plan to find the triply congruent point of highly doped lithium niobate crystals from the combination of defect structure design, first-principle calculation and crystal growth, it will greatly increase the optical quality of highly doped lithium niobate crystals, and then we will fabricate lithium niobate Q-switchers with high resistance to optical damage. This investigation will supply stable and reliable electro-optic Q-switchers for various lasers with high light intensities and extend the practical application of bulk photovoltaic effect.

电光调Q开关是一种广泛用于巨脉冲功率激光的关键器件，在军用激光中已经得到了广泛的应用。铌酸锂电光调Q开关易于加工、无需密封、镀增透膜后透过率高、工作温度宽，具有广泛的应用前景，但光损伤阈值低限制了其在强激光领域的实际使用。项目首次提出铌酸锂晶体光损伤阈值低的原因不仅在于光折变，更在于由体光生伏打效应导致的光折变，提高晶体抗光损伤的关键在于抑制晶体的体光生伏打效应。然而目前铌酸锂晶体光生伏打效应的物理机制尚未搞清，项目拟通过对体光生伏打效应的深入研究，探寻其产生的物理机制，找出有效的控制措施，使铌酸锂晶体的抗光损伤性能有根本性提高；同时，项目拟融合缺陷结构设计、第一性原理计算、晶体生长等方法，找出铌酸锂晶体的三相同成分点，从根本上提高掺杂铌酸锂晶体的光学质量，制作出高抗光损伤电光调Q开关。本项目不仅可以为大功率军用激光器提供稳定可靠的电光调Q开关，还可以进一步开拓体光生伏打效应的实际应用。

第一性原理研究结果表明，铌酸锂晶体最稳定的本征缺陷结构由一个反位铌与两个第一近邻锂空位和第二近邻锂空位组成，掺杂铌酸锂中也存在类似的稳定结构。由于第二近邻锂空位与掺杂离子处于与自发极化垂直的平面内，缺陷偶极矩对晶体自发极化没有直接影响。由此推断，晶格畸变对自发极化的影响可能更大，为调控晶体的光生伏打场指明了方向。.生长了一系列重铁掺杂铌酸锂晶体，其中5.0 wt.％Fe掺杂的铌酸锂晶体的暗电导率和473 nm激光照射时的光电导率达到3.30×10^-8 Ω-1cm-1和1.46×10^-7 Ω-1cm-1，分别比同成分铌酸锂高7个和5个数量级。按照光生伏打的极化子模型，激发概率由Fe_Li^(2+)到周围临近的8个Nb_Nb^(5+)的距离决定，相邻距离越短，沿着该方向的激发概率越大，由于铌酸锂晶体结构的不对称性，导致沿着+c轴产生的净电流最大。提升铌酸锂晶体光生伏打效应的关键在于增大晶体的不对称性，施加应力可以显著提高晶体的光生伏打电流，但更为有力措施是提高掺杂浓度。.在硅基上成功制备了氮掺杂铌酸锂薄膜。霍尔效应测量显示掺氮铌酸锂薄膜呈p型导电，空穴浓度为7.31×10^15 cm-3，场效应迁移率为266 cm2V-1s-1。X射线光电子能谱表明，N原子可能取代了O位，氮原子含量达到0.87％。p型LiNbO3:N薄膜的生长为光伏器件的制作奠定了基础。.按照Li2O-Nb2O5-MgO三元同成分配比的方式生长了系列的同成分高掺镁铌酸锂晶体，实验结果表明，同成分高掺镁铌酸锂晶体中Mg6.0:LN的均匀性最好，明显优于传统二元同成分配比的Mg5.0:LN晶体，且Mg6.0:LN晶体的抗光损能性能也高于Mg5.0:LN晶体。.生长了系列锆镁双掺铌酸锂晶体，研究结果表明，在Mg5.0:LN中仅掺入0.5 mol.% ZrO2就可以将晶体的抗紫外光损伤性能提高近两个数量级，达到2.2×10^5 W/cm2，优于Zr2.0:LN晶体。即使在蓝绿光波段，晶体的光致折射率变化也小于Zr2.0:LN晶体，表明Mg5.0,Zr0.5:LN晶体的抗光损伤性能在2.0×10^7 W/cm2以上。.制备了高抗光损伤阈值、高温度适用性掺镁铌酸锂电光调Q开关，在100Hz高重频纳秒激光系统中的抗光损伤阈值达到210 MW/cm2以上，产生了显著的军事和社会效益。