Investigation of Ultraviolet-Infrared Laser-induced Domain Inversion in Lithium Niobate Crystal

铌酸锂晶体的铁电畴工程通过在微观尺度上对铁电畴结构进行调制，实现铌酸锂晶体中铁电畴的周期极化，已经广泛应用于准相位匹配非线性光学和新型光学器件等诸多重要领域。通常施加周期分布外电场在铌酸锂晶体中制作周期铁电畴，但是反转畴体的尺寸被限制在几个微米左右，很难进一步减小。非线性铁电材料的未来发展，例如一阶准相位匹配紫外二次谐波的产生、电光布拉格光栅和光子晶体等，都需要亚微米和纳米量级的周期反转畴。为了克服这个瓶颈，目前已经研发出很多种新型的铌酸锂晶体极化技术，激光诱导铌酸锂晶体畴反转就是其中很具发展潜力和应用前景的一项新技术，有望在较低的均匀外电场下，通过激光与铌酸锂晶体的相互作用实现微米和亚微米量级的畴反转。因此，深入研究和发展激光诱导铌酸锂晶体畴反转技术是有着十分重要意义的研究课题。 目前激光诱导铌酸锂晶体畴反转的研究已取得一定进展。德国波恩大学K. Buse课题组的研究人员利用紫外激光辐照实现诱导同成分纯净和掺镁铌酸锂晶体畴反转。C. L. Sones等采用可见激光（514，488，和 457 nm）聚焦诱导同成分纯净和掺镁铌酸锂晶体，可以大幅度降低晶体的成核场。V. Dierolf等采用488nm波长激光聚焦诱导直接在同成分纯净铌酸锂晶体表面制作微米量级的反转畴。但是总的来看，激光诱导铌酸锂晶体畴反转研究尚处于初级阶段，仍然是不全面和不系统的，尤其是不同诱导波长和光强对铌酸锂畴反转的影响还没有做任何系统的分析，对激光诱导铌酸锂畴反转的物理机制尚不能提出统一的解释。 本论文针对激光诱导铌酸锂晶体畴反转研究存在的问题，研究了紫外－红外不同波长的激光诱导对不同铌酸锂晶体畴反转的影响，首次系统研究了紫外－红外的不同诱导波长和光强的激光诱导对掺镁铌酸锂晶体成核场的影响，并应用数字全息干涉测量技术研究了铌酸锂晶体的畴反转特性，此外还研究了伴随掺钌铌酸锂晶体畴反转的电致变色效应，得到一系列创新性成果。本论文的主要内容包括以下三个方面： (1) 研究了紫外－红外不同波长的激光诱导对不同铌酸锂晶体畴反转的影响，首次系统研究了紫外－红外（351nm－799nm）波段内不同波长和不同光强的激光诱导对掺镁铌酸锂晶体成核场的影响，发现一定波长和光强的激光辐照可以显著降低掺镁铌酸锂晶体的成核场，首次提出激光与电场在晶体内部的共同作用产生缺陷结构是成核场降低的根源，并提出了有力的实验证据。这项研究成果为进一步开展激光诱导掺镁铌酸锂晶体畴反转的研究提供了重要的实验依据。在此基础上首次对紫外激光诱导近化学计量比钽酸锂晶体铁电畴反转进行了实验研究，并采用数字全息干涉测量技术对激光诱导畴反转区域进行了三维测量，这项研究成果为系统地开展近化学计量比钽酸锂晶体激光诱导畴反转的相关研究提供了可能的方案。 (2) 应用数字全息干涉测量技术研究了铌酸锂晶体的畴反转特性。一方面，应用数字全息干涉测量技术研究了铌酸锂180°畴壁的动态和静态特性，首次发现铌酸锂晶体180°畴壁附近存在位相弛豫，提出极化反转导致180°畴壁两侧附近出现晶格畸变是180°畴壁附近位相弛豫的根源。这项研究成果对铌酸锂晶体180°畴壁物理机制的深入研究提供了新的实验依据；另一方面，应用数字全息干涉测量技术研究了铌酸锂晶体的内电场，首次提出有效内电场的概念，并且针对其物理本质给出合理的解释，这项研究成果对铌酸锂晶体内电场物理机制的深入研究提供了重要的实验依据。 (3) 首次发现了伴随掺钌铌酸锂晶体畴反转发生的电致变色效应，通过不同实验方案分析了掺钌铌酸锂晶体中的畴反转与电致变色效应之间的关系，并提出了相应的物理解释。这项研究成果对进一步研究畴反转和电致变色效应的物理机制提供了新的实验证据。Domain engineering in LiNbO3 is achieved by modulating the domain structure in the microscale and realizing the periodically poling of LiNbO3, which has been successfully exploited for various quasi-phase-matched (QPM) nonlinear applications and novel optical devices. Electric field poling with structured electrodes is routinely employed for the fabrication of periodically poled of lithium niobate (PPLN), but the period lengths obtainable with this technique are currently limited to a few micrometers. However, the future applications of the ferroelectric nonlinear materials such as efficient first-order QPM UV second harmonic generation, electro-optic Bragg gratings, and photonic crystal structures require submicroscale to nanoscale domain periods. Many alternative methods for domain patterning are currently under investigation. Laser-induced domain inversion is a promising technique to overcome the size limitation which exploits the interaction of laser light with ferroelectric lithium niobate to engineer domains with micron to submicron scale at low uniform external field. So the investigation of laser-induced domain inversion is a subject with great importance. Much progress has been made in the investigation of laser-induced domain inversion. Some poling experiments which take advantage of the ultraviolet laser-induced transient change of the coercive field to transfer a patterned light distribution into an equivalent domain structure in the illuminated bulk crystals have already been reported for undoped and MgO-doped lithium niobate crystals by K. Buse et al. An order-of-magnitude reduction in the electric field required for domain nucleation in MgO-doped lithium niobate crystals induced by illumination from a focused continuous wave laser beam at wavelengths of 514, 488, and 457 nm has achieved by Sones et al. The focused visible laser light has recently been used to directly write surface domain with micron dimensions in undoped lithium niobate crystals by Dierolf et al. Howerer the systematic investigation of the wavelength influences on the laser-induced domain inversion has not been performed yet, and the uniform explanation for the origin of the laser-induced domain inversion in the LiNbO3 crystals has not been proposed yet. In this dissertation, the ultraviolet-infrared laser-induced domain inversion in various LiNbO3 crystals is investigated. Within the wavelength range from 351 to 799 nm, the different reductions of nucleation field induced by the focused continuous laser irradiation with varying irradiation intensity are systematically investigated in the MgO-doped congruent LiNbO3 crystals for the first time. The digital holographic interferometry is used to investigate the domain inversion in LiNbO3. Electrochromism accompanying the domain inversion in congruent RuO2-doped LiNbO3 crystals at room temperature are investigated in the experiments. The most work can be presented as: (1) The ultraviolet-infrared laser induced domain inversions in various lithium niobate are investigated. Within the wavelength range from 351 to 799 nm, the different reductions of nucleation field induced by the focused continuous laser irradiation with varying irradiation intensity and wavelength are systematically investigated in the MgO-doped congruent LiNbO3 crystals for the first time. The physical explanation of laser-induced domain inversion in MgO-doped congruent LiNbO3 crystals is proposed, and the solid experimental proof is also proposed. From the results, the solid experimental proofs are supported for the further investigation of laser-induced domain inversion in MgO-doped LiNbO3. The ultraviolet laser-induced ferroelectric domain inversion in nearly-stoichiometric LiTaO3 is also investigated. The local domain inversion induced by the laser radiation area is confirmed by the measurement of the digital holographic interferometry. The results can support the feasible scheme for the further investigation of ferroelectric domain engineering in nearly-stoichiometric LiTaO3 crystal. (2) The characteristics of domain inversion in congruent LiNbO3 are investigated by the digital holographic interferometry. Firstly, the dynamic and static characteristics of 180° domain wall are investigated by the digital holographic interferometry. The existence of phase relaxations adjacent to 180° domain wall in congruent LiNbO3 crystal is investigated firstly. The lattice distortion induced by complex physical interactions during electric poling is proposed to be the source of the effects. From the results, the solid experimental proofs are supported for the further investigation of 180° domain wall in LiNbO3. Secondly, the internal field of LiNbO3 is measured by the digital holographic interferometry, and the effective internal field is firstly proposed. The physical explanation of the effective internal field is proposed. From the results, the solid and important experimental proofs are supported for the further investigation of internal field in LiNbO3. (3) The electrochromism accompanying the ferroelectric domain inversion in congruent RuO2-doped LiNbO3 crystal is investigated at room temperature. The relations between electrochromism and domain inversion are investigated by different methods, and the physical explanation is proposed. From the results, the solid experimental proofs are supported for the further investigation of domain inversion and electrochromism