Fabrication and application investigation of laser induced domain-inversion in LiNbO3 crystals

由于铌酸锂周期性畴极化是激光准相位匹配波长变换技术的重要方案，锂酸锂畴极化技术成为一个国际研究热点。同时，结合妮酸铿晶体特有的光折变、电光、压电、弹光等多种效应，铌酸锂畴极化在光学扫描器、电光开关、光学滤波器等电光器件中的应用也有新的进展。特别是畴极化结构的电控相位体光栅，能够灵活有效的在单块晶体材料中设计多功能的电光器件，在三维光学集成系统中有重要应用。外加电场极化法是目前制作铌酸锂畴极化结构较为成熟的技术，但由于铌酸锂晶体中存在很高的矫顽电场，晶片的厚度和畴结构的周期都受到很大的限制p同时，若要在单块晶体中制作三维畴结构电控局域体光栅以及进一步设计任意形状的电光器件，外加电场极化法还有很大的局限性，越来越不能满足光学应用对周期性畴极化的需求。所以发展适用于在晶体任意位置制作任意形状精细结构的畴反转极化技术具有重要意义。最近国际上报道了一种新的畴结构反转控制技术，在室温下采用紫外激光照射能够显著降低铌酸锂晶体的矫顽电场，从而在较低的均匀电场下就可以实现畴极化反转的局域选择性。这一技术方案不但能够为进一步采用激光相干条纹照射制作较厚和更小周期的畴结构提供了新的途径，而且最重要的是将成为在单块晶体任意位置制作任意形状精细结构的三维畴结构电控局域体光栅和电光集成器件非常实用的有效方案。所以，深入研究和发展紫外激光诱导电场极化法具有创新意义和重要应用前景。本论文主要研究了铌酸锂晶体在紫外激光敏化诱导下的畴反转特性，在此基础上设计了畴结构电控局域体光栅，同时对双掺杂铌酸锂畴反转过程发现的现象一电色效应进行了分析。主要研究结果如下：1、在国内首次成功实现了同成份比铌酸锂晶体的紫外激光诱导畴反转。研究表明，通过控制紫外激光的照射区域可以有效的实现畴反转的局域可控性，这不但在制作精细周期性畴结构中有重要应用，而且将发展成为在晶体内部任意深度制作任意边缘形状畴结构的有效技术方案。2、基于铌酸锂晶体紫外激光诱导畴反转技术，设计了一种畴结构电控体光栅，并分析了实现这种电控体光栅记录和再现的原理。该体光栅充分利用了铌酸锂晶体较大的电光系数的特性，通过控制外加电场实现晶体内部折射率调制的实时性和局域可控性。3、观察到了双掺杂LINb03：Ru：Fe晶体畴反转过程中发生的电色效应，发现该效应与畴反转同样具有可逆性。实验结果证明，电色效应和畴反转是相辅相成的，即畴反转导致了晶体变色，电色效应促进了畴反转，并基于铁电微结构模型，简要解释了其机理。同时还发现该种掺杂晶体不同于传统的妮酸铿晶体，在低于击穿电场的条件下就能够实现畴反转，从而用恒定直流电场代替脉冲电场就能实现畴反转，这一特性将对简化实验装置和操作有积极意义。Lithium niobate domain-inversion technology becomes a global research hot pot, because it is the key technology of quasi-phase-matched wavelength transformation. Combining multiple properties of LiNbC>3, including photorefractive effect, electro-optic effect, piezo-electricity and elasto-optic effect, the new applications develop greatly in electro-optical devices, such as optical scanner, electro-optic switch and optical filter. Especially, the domain structure electro phase volume grating can be used for multi-function electro-optic devices in single piece of crystals flexibly, so there are many important applications in 3D optical integration systems. The external electric poling method is a mature technique of fabrication of domain-inversion currently, but the crystals thickness and domain structure periods are limited because of the high coercive field of LiNbCb. It has much limitation, if a 3D domain structure electro local volume grating is designed in a single piece of crystals, and arbitrary shape electro-optic devices. Therefore, it cannot satisfy the demand in optical field. It is very important that a new domain-inversion technology is developed to design a single piece of crystals with any positions, any shapes and fine structures. Recently, a new technology, ultraviolet induced domain-inversion, has been invented overseas. For the characteristics of ultraviolet effectively reducing the coercive field of LiNbO3, ultraviolet can control the domain-inversion area. This technology provides a new method for thicker and finer period domain structure, furthermore, it will become an effective scheme for arbitrary shape and fine structure 3D domaia structure electro local volume grating and electro-optical integration devices. Hence it is creative and important that ultraviolet induced domain-inversion need to be researched profoundly. We investigate the characteristics of this technology in LiNbCh, and design a domain structure electro local volume grating, and discover a new phenomenon, electrochromic effect, in domain-inversion poling process in doubly doped LiNbC>3 crystals. Main research results are following: We realize ultraviolet laser induced domain reverse in LiNb(>3 Crystalssuccessfully experimentally for the first time in China. The investigation proves thatdomain-inversion local controllability can be realized by controlling illuminate area,and it won't only be applied to fabricating finer domain structures, but it will alsobecome a new method to creating an arbitrary shape domain structures in anydeepness in LiNbCb crystals. Based on the ultraviolet induced domain-inversion technology, we design adomain structure electro volume grating, and analyze the theory of holographic recordand representation. The volume grating makes the best of the largest electro-opticcoefficient, and the refractive index can be modified by controlling electrical field torealize the real-time and local controllability. It has much applicative foreground in3D integration optics system. A reversible electrochromic effect accompanying domain-inversion duringthe electrical poling process in LiNbC>3:Ru:Fe crystals at room temperature isobserved. In electrode area, both electrochromism and domain-inversion occurmutually, and electrochromism is also reversible during back-switch poling, which areexperimentally verified and whose mechanism are briefly explained based on a micro structure ferroelectric model. In addition, different from the undoped LiNbC>3 crystals, the coercive field measured in LiNbO3:Ru:Fe is lower than its breakdown field, which proves a possible technique to realize domain-inversion by constant electric field rather than pulsed one. It must simplify the experimental setups and operations