【作者】 任相魁；

【导师】 宋峰； 张天浩；

【作者基本信息】 南开大学 ， 凝聚态物理， 2010， 博士

【摘要】 光折变表面波是一种可沿光折变非线性介质表面传播的光波,它能将光能量约束在光折变介质近表面狭层空间。其高能量密度的特点和可沿直线传播而实现的相位匹配,使各种表面非线性光学效应得以增强。因此,光折变表面波在波导制备、谐波产生和集成光学器件设计等方面具有潜在的应用价值。尽管人们对光折变表面波已经有了较深入的了解,但对光折变表面波的产生机制、模式调控及其稳定性等很多问题都还需要做进一步的研究。此外,随着双光子光折变晶体以及聚合物和液晶等新型光折变材料的发展,材料的光折变性能也比传统的无机晶体有了长足的提高和优化。本论文围绕光折变表面波相关问题作了系统的研究,主要内容包括：(1)研究了光折变表面波的产生机制,较系统地分析了光折变效应的三种非线性机制,扩散、漂移和光生伏打效应,对光折变表面波的作用。指出扩散非线性是光折变表面波形成的本质原因,漂移非线性和光伏非线性并不是光折变表面波形成的必要因素,但是可利用它们对光折变表面波模式进行调控。(2)研究了光折变表面波的激发和模式特征,给出了光折变表面波的模式与光波入射角和传播常数的关系,较大的入射角对应较大的传播常数和低阶模式的表面波,较小的入射角对应较小的传播常数和高阶模式的表面波。(3)分析了光折变表面波的稳定性。建立了等效振子模型并利用该模型实现了对光折变表面波稳定性的直观定量分析,指出外加电场存在一个阈值Eth=(k02n2-β2)/k02n4reff,如果电场高于此阈值,光折变表面波的稳定性将会被破坏同时利用快速傅里叶变换光束传播理论对稳定性进行了测试。(4)研究了双光子光折变晶体中光折变表面波的形成机制与特点。单光子光折变表面波所诱导的表面波导可以方便地被擦洗掉,但同时这种可擦洗性也使得表面波导存在不稳定性因素,不利于长期保存。由于双光子光折变效应特殊的两步激发机制,双光子光折变表面波可以诱导出不可擦洗的表面波导,这为表面波导永久性存储问题的解决提供了有效方案。(5)系统地研究了聚合物和液晶光折变材料中的光折变表面波。相对无机光折变晶体,聚合物和液晶光折变材料具有成本低廉、结构可控、工作波长范围宽、易于加工等优点,而且其光折变性能也比无机光折变晶体高出很多。我们建立了聚合物和液晶光折变表面波的理论分析模型,研究结果表明外电场不仅可以调节光折变表面波的空间频率,还可以有效地控制其贯穿深度,而且聚合物和液晶光折变表面波还具有较独特的偏振依赖性,比起无机光折变表面波更加灵活可控。此外,液晶光折变表面波的激发和调控所需的外加电场很低。以上特点使光折变表面波的性能得以提高,并有望得到更广泛的实际应用。本论文对光折变表面波进行了系统性的研究,创新点如下：1.首次建立了光折变表面波的等效振子模型,证明了扩散机制是形成光折变表面波的本质原因,漂移非线性和光伏非线性不是光折变表面波形成的必要因素,但是可利用它们对光折变表面波模式进行调控。给出了光折变表面波的模式与入射角和传播常数的关系,并提出了基于等效振子模型的稳定性分析方法,可定量分析外电场对表面波稳定性的影响。2.首次提出在双光子光折变介质中可以形成光折变表面波,从而诱导出可长期保存的表面波导。3.首次建立了有机聚合物和液晶光折变表面波的理论分析框架,并从理论上证明了在液晶光折变材料表面可以形成光折变表面波。由于聚合物和液晶材料特殊的取向增强光折变效应,外加电场不仅可以改变聚合物光折变表面波和液晶光折变表面波的空间频率,还可以有效地控制其贯穿深度。更多还原

【Abstract】 The photorefractive surface waves (PR SWs) are a kind of waves which can propagate along the surface of the photorefractive materials. The most attractive feature of PR SWs is that the concentration of beam energy with high intensity in the narrow surface layer, which ensures various nonlinear surface optical phenomena to be enhanced. So PR SWs are potentially useful for waveguide fabrication, harmonic generation, integrated optics technique and so on.Although some elementary research about PR SWs has been done, there are still many problems, such as the formation mechanism, mode modulation, and stability of PR SWs, need to be studied further. In addition, with the development of new photorefractive materials, such as two-photon photorefractive crystal, photorefractive polymer and liquid crystal, the photorefractive performance has been improved dramatically. In this dissertation, the PR SWs are studied systematically. The main contents are shown as follows:(1) The formation mechanism of PR SWs is studied. The influences of diffusion, drift, and photovoltaic effect on the PR SWs are analyzed. The analytical results show that:diffusion nonlinearity is the essential cause for the formation of PR SWs. The drift and photovoltaic nonlinearity are not necessary in the formation of PR SWs；but they can be used to modulate the modes of PR SWs.(2) The generation and mode characteristics of PR SWs are analyzed; the relationship between the modes of PR SWs and the incident angles or propagating constants was presented：the large angles of incidence correspond to the large propagating constants and low-frequency modes, while the small angles of incidence correspond to the small propagating constants and high-frequency modes.(3) The stability of PR SWs is analyzed. The equivalent oscillator model is presented and used to do quantitative analysis of the PR SWs’stability. The analytical results show that：the electrical field has a threshold as Eth=(k02n2-β2)/k02n4reff, above which the PR SWs will be destructed. The spatial-partition fast Fourier transform beam propagation theory is used to test the stability of PR SWs.(4) The formation and properties of PR SWs in two-photon photorefractive medium are studied. The surface waveguides induced by single-photon PR SWs can be erased and recorded easily. However, the recording reversibility also makes them volatile and can not be stored for long period. Due to the two step photorefractive mechanism, the two-photon PR SWs can induce surface waveguides with no erasure problem, which supplies a feasible method for creating permanent surface waveguides. (5) The PR SWs in photorefractive polymer and liquid crystal are studied systematically. Organic photorefractive materials can provide many advantages over the traditional inorganic crystals, which include cheapness, flexibility of fabrication, wide working wavelength range, ease of processing, and improvement of PR effect. In this dissertation, the theoretic models for PR SWs in polymer and liquid crystal are presented, the analytical results show that：the applied external electric field can change not only the spatial frequency but also the penetration depth of the polymeric and liquid crystal PR SWs. Moreover, the properties of PR SWs in polymer and liquid crystal are polarization-dependent. Additionly, the PR SWs in liquid crystal can be excitated and modulated with much low electric field. These properties make the PR SWs in polymer and liquid crystal more controllable than that in inorganic photorefractive crystals. By taking advantage of the above characteristics, the performance of PR SWs can be improved dramatically and used widely.In this dissertation, the PR SWs are studied systematically. The innovative points are as follows:1. The equivalent oscillator model is presented for the first time. It is shown theoretically that the diffusion nonlinearity is necessary in the formation of PR SWs, while the drift and photovoltaic nonlinearity are not essential causes for the formation of PR SWs；but they can be used to modulate the modes of PR SWs. The relationship between the modes of PR SWs and the incident angles or propagating constants was presented. The method based on equivalent oscillator model to anlyze the stability is presented and can used to analyze quantitatively the influence of the applied external electric field on PR SWs.2. We predict that the PR SWs can be supported by two-photon photorefractive medium. The two-photon PR SWs can induce surface waveguides that can be stored for long period.3. The theoretical framework for PR SWs in polymer and liquid crystal is presented. It is shown theoretically that PR SWs can form in photorefractive liquid crystal. Due to the orientationally enhanced photorefractive effect, the applied external electric field can change not only the spatial frequency but also the penetration depth of the PR SWs in polymer and liquid crystal.更多还原