【作者】 王文杰；

【导师】 孔勇发；

【作者基本信息】 南开大学 ， 光子学与光子技术， 2011， 博士

【摘要】 随着激光的出现,人们对于光学的认识发生了重要的变化。当一束激光入射到介质以后,会从介质中出射一束或几束有新频率的光束,人们把这种效应称为非线性效应。光学频率变换是激光及非线性光学领域的一个重要分支,如利用材料的非线性效应可实现二次谐波、三次谐波的产生。频率的转换效率依赖于非线性作用过程中的相位匹配因子。利用材料的双折射性质可以实现非线性作用过程中的相位匹配,然而它受材料的双折射性大小、温度、作用波长及偏振等方面的限制。1962年,Armstrong[1]等人提出通过周期地改变材料的二阶非线性系数可以增强非线性作用过程中的频率转换效率,即准相位匹配技术的产生,这样的晶体结构称为周期极化结构;1993年,Yamada等人[2]首次报道了利用外加电场极化法在铌酸锂晶体中制成了一维的周期极化结构。近年来,人们发现利用激光辐照的方法可以降低铌酸锂晶体极化反转过程中的外加电场。这一发现不仅使得制备周期极化铌酸锂晶体的难度进一步减小,有利于获得比目前更厚的晶体样品,而且为畴结构的制备提供了一种新的方法。一维周期极化结构往往会限制非线性作用过程中多个频率的变换。1998年,Berger等人[3]提出了非线性光子晶体的概念,把准相位匹配技术扩展到二维的情况,可以同时实现多个频率的变换。基于二阶非线性系数周期变化的非线性光子晶体是一种极具发展前景的光波长变换材料;而根据二阶非线性系数的变化形式,可以实现准周期甚至无序形式的非线性晶体结构。当激光与这些材料相互作用时,可以同时实现多个非线性相互作用的过程,如宽带的倍频及级联三次谐波的产生。本论文的内容包括以下几个方面:第一章对材料的二阶非线性效应及二次谐波、三次谐波的产生过程进行了简单的介绍,概括的介绍了准相位匹配技术的原理、制备方法等,并给出了本论文的工作安排。第二章研究了可见光聚焦辐照下铌酸锂晶体的光诱导铁电畴反转特性。首先简介了铌酸锂晶体的极化特性,畴反转过程及畴结构的观测方法等。随后,我们在实验和理论上对可见光诱导铌酸锂晶体中畴反转电场降低的机理进行了深入的研究和探讨。我们研究了不同掺杂的铌酸锂晶体在514.5nm可见光聚焦辐照下畴反转电场降低的情况。我们发现,对于不同掺杂的晶体,反转电场降低的最大数值基本相同,并且极化畴总是在晶体的-c面首先成核;不同于一般情况下得到的反转畴结构,用激光诱导方法形成的畴结构十分稳定。通过分析,我们建立了空间电荷场的理论模型,合理的解释了激光诱导铌酸锂晶体畴反转的过程。第三章在实验及理论上研究了无序铁电畴结构的铌酸锶钡(SBN)晶体中二次谐波及级联三次谐波的产生过程。首先,分析模拟了无序畴结构的倒格矢空间,由于倒格矢量的连续性,使得倍频及和频过程中的相位匹配关系可以同时得到满足,从而在空间域及频域大大增加了谐波的调节范围;随后,我们进行了双光束耦合实验,通过测量和分析二次谐波能量在空间的分布形式等,可以实现短脉冲信号的测量,如脉宽、脉冲序列等;最后,我们首次从实验上发现,在无序畴结构的SBN晶体中同样可以实现级联三次谐波的产生,并对它的产生过程及偏振性质等进行了详细分析,在理论上模拟了谐波的产生过程,表明谐波的能量随着传播距离的增加是一个线性增长的过程。第四章研究了周期及准周期极化结构中非线性的Cerenkov和Raman-Nath衍射。首先,介绍了二次谐波的Cerenkov及Raman-Nath两种非线性衍射形式,着重分析了Cerenkov衍射环的能量分布及偏振特性;随后,我们通过双光束耦合,首次在实验上观察到了多组二次谐波Cerenkov衍射环,包括虚拟Cerenkov衍射环的产生,对它们的空间能量分布、准相位匹配关系及偏振性质等进行了详细分析;最后,我们实验上首次发现了多个三次谐波衍射环的产生,对它们的产生机理、过程、空间能量分布等进行了深入的研究,发现三次谐波衍射环的产生是基于Cerenkov及Raman-Nath型二次谐波的级联效应。第五章对本论文的工作进行了总结和展望。更多还原

【Abstract】 As Laser showed up, people’s knowledge towards optics was dramatically changed. Nonlinear optics became one of the interest subjects when laser interacts with some media with one or more new frequency generated, known as frequency conversion processes, such as sum frequency, difference frequency effects. The efficiency of frequency conversion depends on the phase matching conditions, which can usually be fulfilled by the Birefringence method, while the materials dependence, temperature, input wavelength, and polarization limit its further applications.The concept of Quasi-Phase-Matching (QPM) with periodic modulation of the sign of second-order nonlinear susceptibilityχ(2) was originally suggested by Armstrong and the coworker in 1962. Since then, QPM became an issue of material engineering known as periodically poled crystals, which was successfully fabricated in one-dimensional (1D) lithium niobate crystal by electric poling method by M.Yamada in 1993. While, recent year’s studies show that the laser-assisted illumination can decrease the poling electric field of lithium niobate crystal which can make the poling process much easier and also can get much thicker samples, therefore the possibility of light-assisted domain engineering opens up a promising method for domain structuring.1D periodic structure is limited in simultaneous multi-frequency conversion. The conception of nonlinear photonic crystal was proposed by V.Berger in 1998 by employing a two-dimensional (2D) periodic modulation ofχ(2) in plane, known as 2D QPM, which can simultaneously phase match multi-second-order frequency conversion process. Nonlinear photonic crystal is a promising structure in nonlinear frequency conversion, while based on the modulation style ofχ(2), quasi-periodic and even disordered photonic structures can also be achieved, presenting a broad application, such as broadband second-harmonic and cascaded third harmonic generation.As my PhD thesis, the following contents are included in this paper: In the first chapter, firstly, the second-order nonlinearity is shortly discussed followed by a section about the second- and third-harmonic generation process. Then, it gives a short introduction to the theory and engineering method of QPM. In the end, the general idea about the research subjects in my PhD thesis is presented.As the second chapter, the focused-light-induced ferroelectric domain inversion in lithium niobate crystal is deeply investigated. Firstly, the general poling properties and poling process of ferroelectric domains in LN and the corresponding detection method are included. Then, with an emphasis, the origin of light-induced decrease in electric field for domain inversion in LiNbO3 crystals with different dopants are invesigated experiementally and theoritically with focused 514.5nm laser beams. It is found that the light-induced maximum values of electric field for domain reversal are almost the same. Besides, inverted domains always first nucleate on the– c surface and behave very strong stability within the illuminated region. Based on these experimental results and analysis, a qualitative space-electric-filed mode is proposed as a reasonable explanation on the light-induced domain reversal process. In the end, a few methods people using for domain fabrication with the assistance of light-illumination was summarized as a promising way in domain engineering.In the third chapter, the second-harmonic (SH) and third-harmonic (TH) gene- ration process in strontium barium niobate (SBN) crystal with disordered ferroelectric domains is studied. Firstly, the disordered domain structures can provide a continuous reciprocal space to phase match the harmonic generation process, such as SH and TH, resulting in broadband harmonic generation in space and frequency spectrum. Then, based on the two-coupled-beams experiment, by analyzing the intensity distribution of SH in space, the implementation for characterization of short-pulses is demonstrated, such as enabling the estimation of pulse width, pulse sequence and so on; In the end of this chapter, it is stressed that we demonstrated firstly in the experiment that the disorder allows the realizing of broadband TH generation, followed by the analysis of the quasi-phase-matching conditions and polarization properties. The harmonic generation process is studied numerically showing that the harmonic intensity increases with the propagation distance.In the fourth chapter, the nonlinear Cerenkov and Raman-Nath diffraction is studied in quasi- and periodic nonlinear photonic structures. Firstly, we introduce the conception of these two kinds of nonlinear diffraction and the corresponding phase-matching condition followed by an emphase on the analysis of intensity distribution and polarization properties of Cerenkov diffraction. Then, based on the two-beams-coupled experiment, multi-Cerenkov second harmonic rings are firstly obtained in experiment including the observation of the virtual one, followed by the analysis of the corresponding nonlinear process and the intensity distribution. Finally, it is worth stressed that it is the first time the multi-TH rings can also be generated in the experiment. Based on the detailed analysis of the corresponding nonlinear processes and intensity distribution in space, we demonstrated that the TH generation is a second-order cascaded process based on the Cerenkov and Raman-nath emission of SH generation.As the last chapter, we give a summarization of the previous work and the prospect for the further research.更多还原