【作者】 邓学伟；

【导师】 陈险峰；

【作者基本信息】 上海交通大学 ， 光学， 2011， 博士

【摘要】 铁电晶体因其较好的非线性性质以及自发极化能够被外加电场反转的特点而在非线性光学的研究中有着广泛的应用。在研究铁电畴和畴反转的过程中,人们发现畴壁作为一个特殊的界面,在光学、电学、力学等方面表现出许多奇特的性质。其中一部分性质已经通过第一性原理计算和动力学模型得到了较好解释。尽管如此,在非线性性质方面,人们对畴壁的研究才刚刚起步,相关报道也不多见。我们的工作正是基于铁电畴及畴反转结构,针对畴壁新的非线性性质开展研究。另一方面,也对铁电畴及畴反转结构在电光控制方面进行了探索。在对铁电晶体的基本性质进行了简单介绍后,我们回顾了国际上常用的畴反转制备方法,其中室温电场极化法应用最为广泛。我们在实验室中进行了室温电场极化实验,成功制备了周期性极化反转以及大面积极化反转。这两种类型的畴反转结构是我们后续研究的基础。通过对铁电晶体畴壁激发切伦科夫倍频的详细分析,我们发现铁电晶体畴壁上存在巨大提升的非线性性质。利用这一性质,我们提出了一种新的扫描光学成像的方法——利用畴壁切伦科夫倍频信号重建畴壁形态。该方法具有实时、无损、高精度及可三维成像的优点。利用该方法对畴壁厚度的估算在10nm量级,空间角分辨率达10mrad。由于畴壁对基频光场响应的定域性,它会对其上产生的非线性极化波产生调制。随着基频光与畴壁间夹角的增大,非线性极化波的相速度会被调制变快。这直接影响了切伦科夫倍频的产生,改变了倍频光的出射角。我们据此分析了在反常色散中由畴壁调制作用产生切伦科夫倍频的可能,并通过实验加以证明。通过对其原理的分析,我们将该现象命名为非线性Smith-Purcell效应。基于畴壁对切伦科夫倍频的调制作用,我们设计了畴壁序列产生完全位相匹配倍频的实验。这种类型的倍频是与准位相匹配机制不同的全新机制。实验中产生倍频的归一化效率超过了目前报道中准位相匹配技术产生倍频的最高效率。产生如此高效倍频的原因是畴壁巨大的非线性系数提升以及完全位相匹配的实现。畴壁序列产生完全匹配高效切伦科夫倍频的方案具有实用意义,可用于激光系统中。基于铌酸锂晶体的电光效应,我们提出了外加电场控制周期性极化铌酸锂波导阵列中离散空间光孤子产生与坍缩的方法。理论模拟表明,改变外电场不仅能够促使孤子形成和坍缩,也能改变波导阵列输出端面上的能量分布。实验证实了施加z向外电场的周期性极化铌酸锂晶体中可以形成离散空间孤子。改变外电场强度,可以控制它的产生和坍缩。利用钽酸锂晶体较大的电光系数,我们设计并制作了准速度匹配电光相位调制器。我们完全掌握了器件的理论设计和制造工艺,制作出不同通光口径的调制器。测试结果表明,调制器能够有效展宽入射光频谱,符合设计标准。更多还原

【Abstract】 Ferroelectrics, which have good nonliearities and can be inverted by external electrical field, have been widely used in nonliear optics. During the researches on ferroelectric domain and domain inversion, people found a lot of new characteristics of optical, electrical and mechanical properties on their interface—domain wall. Some of these new characteristics can be well explained by the first-priciple calculations and dynamic models. However, in the nonliear aspect, people are just starting to look at domain wall’s nonliearities, and few report is found. Our work here are based on ferroeletric domain and domain inversion, aimed at domain wall’s nonliearities. And we also make study on the electro-optical applications of domain inversion at the same time.After simple introduction of ferroelectrics, we review the common techniques to produce domain inversions, among which the poling at room temperature by external electrical field technique is the most widely used. We produce periodically poled and bulk domain inversion by this technique under lab environment. These two types of domain sturctures are the basis of our work here.Through a detailed anylasis of Cherenkov second harmonic generation (CSHG), we found significant enhancement of nonlinearities in domain wall. Based on this enhancement, we proposed a new method of scanning optical microscopy—reconstruct the domain walls by collecting the signals of CSHG in domain wall. This method has advantages of in situ, nondestructive, high resolution and 3-D reconstruction. We evaluated the width of domain wall is at the magnitude of 10nm and got a spatial angular resoluton of 10mrad.Domain wall can modulate the nonlinear polarization in it by locality. With the increasing of incident angle, the phase velocity of nonlinear polarization can be modulated faster. It affects the CSHG’s ejecting angle. Based on domain wall’s modulation effect, we predicted a CSHG in nonlinear material with anomalous dispersion and demonstrated it experimentally. Based on its principle, we named this effect nonlinear Smith-Purcell effect.Also based on domain wall’s modulation effect, we designed the complete-phase-matching CSHG experiment. This is a new mechanism distinguished from quasi-phase-matching technique. The normalized conversion efficiency was even higher than that of quasi phase matching technique. It is because the giant enhancement of nonlinearities in domain wall and the complete-phase-matching mechanism. This scheme has application meaning, and can be used in laser systems.We proposed a method to control discrete spatial soliton and its collapse in periodically poled lithum niobate (PPLN) by electro-optical effect. Theoretical simulations showed that external electrical field can control the formation and collapse of soliton and change the energy distribution on the output surface. Experiment demonstrated that solitons can be formed in PPLN with z-directional electrical field. By changing the electrical field, one can control the soliton’s formation and collapse easily.We also produced a quasi-volocity-matching electro-optic phase modulator in LiTaO3 crystal by using its large electro-optic coefficient. We made deep understanding on the theorectial design and producing technology. Modulator with different apertures are produced. By testing their modulation indices, we found these modulators can broaden the spectrum of the incidence effectively and rightly meet the design requirements.更多还原