【作者】 马德才；

【导师】 王彪；

【作者基本信息】 哈尔滨工业大学 ， 工程力学， 2007， 博士

【摘要】 铌酸锂和钽酸锂晶体具有优良的电学及光学性能,已经在全息存储和准相位匹配等方面得到了广泛的应用。然而铌酸锂和钽酸锂晶体在高光强照射的情况下会产生光致散射,导致全息存储图像质量严重下降。同时铌酸锂和钽酸锂晶体高的矫顽场使其很难在常温下实现周期极化,使周期极化工艺变得复杂。本文采用掺杂ZnO的方法提高晶体的抗光致散射能力和降低晶体的矫顽场。本文采用最优的工艺参数,利用提拉方法,从中频加热炉中生长出无明显宏观缺陷、光学均匀性好的同成份Zn:LiNbO₃和Zn:LiTaO₃单晶。生长出晶体的尺寸为Φ40×20mm。通过气相交换平衡（VTE）技术制备了近化学计量比铌酸锂和钽酸锂晶片。X射线粉末衍射实验结果表明同成份Zn:LiNbO₃和Zn:LiTaO₃晶体掺Zn后晶格结构未有变化。对Zn:LiNbO₃和Zn:LiTaO₃晶体的晶胞参数进行计算后发现,晶格常数随着ZnO含量的增加而增加。Zn:LiNbO₃或Zn:LiTaO₃晶体的紫外吸收边和红外吸收峰的变化机理也在本文中被讨论。分析显示LiNbO₃晶体中掺入Zn2+离子优先取代反位铌（ Nb L4+i）。当ZnO含量达到其阈值浓度后,所有的反位铌被完全取代,Zn2+离子开始同时占据正常的锂和铌位,形成（ Zn 3N-b-3 Zn +Li）自补偿结构。而对于LiTaO₃晶体,当掺入ZnO掺杂量小于阈值浓度时,Zn2+首先取代反位钽（ Ta L4+i）的位置,当超过阈值浓度后Zn2+开始取代正常晶格的Li位。利用差热分析测试Zn:LiNbO₃和Zn:LiTaO₃晶体晶体的居里温度,发现当ZnO含量低于其阈值浓度时,晶体的居里温度随ZnO含量的增加而上升。相反地,当ZnO含量达到其阈值浓度后,其居里温度开始下降。测量了Zn:LiNbO₃和Zn:LiTaO₃晶体的铁电性能,自发极化对ZnO含量变化不敏感。电滞回线测试结果呈现明显的不对称性,即晶体存在内场。通过同成份与化学计量比LiNbO₃和LiTaO₃晶体的矫顽场和内场相比较可以得出,同成份晶体中高的矫顽场和内场是由晶体的本征缺陷造成的。晶体的矫顽场和内场对ZnO含量的变化十分敏感。利用二波耦合实验测试了同成份和近化学计量比Zn:LiNbO₃和Zn:LiTaO₃晶体的光折变性能,如衍射效率、光折变响应时间及擦除时间。实验结果表明随着ZnO含量的增加,晶体的光电导增加,晶体的衍射效率降低,光折变响应速度变快,同时擦除时间也缩短。近化学计量比晶体的衍射效率要低于同成份晶体衍射效率,光折变响应时间及擦除时间也小于同成份晶体。利用透射光斑畸变法测试了Zn:LiNbO₃和Zn:LiTaO₃晶体的抗光致散射能力。纯的LiTaO₃要比LiNbO₃晶体抗光致散射能力高二个数量级。当ZnO掺杂进入晶体后,掺杂量低于其阈值浓度时,晶体的抗光致散射能力变化不大。相反地,当ZnO掺杂量达到其阈值浓度后,晶体的抗光致散射能力显著增加,与同成份晶体相比提高了二个数量级。近化学计量比铌酸锂和钽酸锂晶体的抗光损伤性能要明显优于同成份的铌酸锂和钽酸锂晶体。分析表明光电导是晶体抗光致散射性能变化的重要原因。更多还原

【Abstract】 Lithium niobium and Lithium tantalum, due to excellent electrics and optical properties, have been widely applied to volume holographic memory. However, light scattering can be induced by high light intensity in lithium niobium and lithium tantalum, which results in serious holograph distortion. simultaneously, because of its high coercive field, periodic polarization wasn’t achieved at room temperature in LiNbO₃ and LiTaO₃ crystals. In this dissertation, in order to improve the resistant ability to optical damage and coercive field, ZnO were doped into LiNbO₃ and LiTaO₃ .In this dissertation, Zn:LiNbO₃ and Zn:LiTaO₃ single crystals were grown by the Czochraski method with intermediate frequency heating. The optimum technological parameters were adopted. There were no macroscopic defects and good optical homogeneity in the as-grown boules. Size of as-grown boules was aboutΦ40×20mm. Vapor transport equilibration method was used to fabricate the near-stoichiometric crystal.X-ray powder diffraction experiment showed that lattice structure of the Zn:LiNbO₃ and Zn:LiTaO₃ crystals was similar to that of the congruent sample. In Zn:LiNbO₃ and Zn:LiTaO₃ crystals, results of unit cell parameters indicated that the lattice constant increased with the ZnO concentration increasing. The change mechanism of the UV-Vis absorption edge and the IR absorption peak was also discussed in the Zn:LiNbO₃ and Zn:LiTaO₃ single crystals.Analysis results showed that Zn2+ ions took priority of replacing anti-site Nb （ Nb L4+i）. When the ZnO concentration reaches the threshold concentration, all of the Nb L4+i ions were completely replaced, and Zn2+ ions begin to occupy the normal Li and Nb sites simultaneously, thus self-compensation defect structure （ Zn 3N-b-3 Zn L+i） was formed. In LiTaO₃ crystals, Zn2+ ions replace anti-site Ta （ Ta L4+i）. All of the Ta L4+i were completely replaced when ZnO concentration reached the threshold concentration, and Zn2+ ions had to replace the normal Li site.Curie temperature of the Zn:LiNbO₃ and Zn:LiTaO₃ crystals was obtained from DTA measurement. Curie temperature increased with ZnO concentration increasing when ZnO concentration was below the threshold concentration. However, when ZnO concentration was over the threshold concentration, the curie temperature began to decrease.Z-cut Zn:LiNbO₃ and Zn:LiTaO₃ were polished to make polarization hysteresis loop measurements. The spontaneous polarization was insensitive to the concentration of ZnO. The hysteresis loops of all samples were asymmetric due to existence of the internal field. The influence of ZnO content on the coercive field and internal field was investigated. The high coercive field and internal field in the congruent crystal were resulted from high concentration of the anti-site Nb and anti-site Ta in comparison with near-stoichiometric crystal.The photorefractive properties of the Zn:LiNbO₃ and Zn:LiTaO₃ crystals, including diffraction efficiency and photorefractive response time, were obtained by two-beam coupling experiments. With the ZnO concentration increasing, diffraction efficiency and erasure time decreased, and the photorefractive response speed increased. In near stoichiometric LiNbO₃ and LiTaO₃ crystals, diffraction efficiency and erasure time were lower than those of congruent crystals.The light scattering resistance of the Zn:LiNbO₃ and Zn:LiTaO₃ crystals was studied by the transmission facula distortion method. When the ZnO concentration was below threshold concentration, the light scattering resistance of samples showed no obvious change. In contrast, when the ZnO concentration reached threshold concentration, the light scattering resistance of samples increased sharply. The light scattering resistance was two orders of magnitude higher than that of pure LiNbO₃ and LiTaO₃ crystals. The light scattering resistance of the near-stoichiometric LiNbO₃ and LiTaO₃ crystals is better than that of congruent crystals. Analysis results indicated the photoconductivity was mainly responsible for the light scattering resistance.更多还原