【作者】 李亚琴；

【导师】 吴世法； 简国树；

【作者基本信息】 大连理工大学 ， 光学工程， 2009， 博士

【摘要】 随着激光的发现、微弱信号探测技术的提高和计算机的应用,拉曼光谱检测技术在很多领域的应用都得到了迅猛的发展。已发展的有傅里叶变换拉曼光谱技术、激光共振拉曼光谱技术、共焦显微拉曼光谱技术、高温拉曼光谱技术、固体光声拉曼光谱技术以及表面增强拉曼散射光谱技术等。其中,表面增强拉曼散射光谱检测技术作为一种最有效的增强拉曼检测信号的手段引起了人们极大的关注,高灵敏度使得这种检测技术在材料、化学、生物、医学等应用领域都具有不可替代的优势。本文围绕对表面增强拉曼散射中的关键问题一金属纳米结构在电磁场激励下的近场电磁增强效应的数值模拟展开,旨在从理论上认识表面增强拉曼散射现象中的电磁增强效应机理,同时也为实验中能够获得更为显著的表面增强拉曼散射效应提供理论依据。本论文内容主要分为四个部分:一、利用处理色散介质的时域有限差分(FDTD)方程来模拟金属纳米结构与电磁场的相互作用问题:描述金属自由电子的模型—Drude模型引入金属介质中的Maxwell方程,得出金属色散介质中的时域有限差分方程;利用多层介质系统中的入射波设置方法—等效入射波法来解决近场散射中隐失波作为激励源的入射场设置问题。得出了完整的可以模拟具有任意形状的金属纳米结构与电磁场相互作用的时域有限差分方程,并在此基础上自己编写了程序,且对其有效性进行了验证。二、利用我们自己编写的程序对不同结构的金属纳米结构在不同激励下产生的近场增强分布进行了数值模拟。利用推导得出的金属中的时域有限差分方程—(FD)~2TD模拟了单个三角形银粒子、双球形银粒子以及银球多粒子聚集体在平面波和隐失场激励下的近场增强分布,结合金属介质中的电子自由程、介电常数、等离子体频率等物理参量,对不同的金属纳米结构在外部激励场下产生的“热点”中的近场增强因子与该结构形状之间的依赖关系进行了解释,从而分析了金属纳米结构的表面增强拉曼散射效应中的电磁增强效应。数值模拟结果表明,金属纳米结构的光学属性、形状、大小、聚集程度以及粒子聚集体的排列方向与激励波的偏振方向之间的关系都直接影响着表面增强拉曼散射实验中样品拉曼信号的强度。利用双球形银粒子以及银球多粒子系统在平面波激励下产生的近场增强分布的数值模拟结果,对Nie等人在单分子拉曼信号探测实验中遗留的几个问题进行了分析,也为实验中选择可以获得更大的局域电场增强效应的金属纳米结构可提供参考。三、为吴世法教授的超高灵敏近场表面增强拉曼散射样品池发明专利概念设计进行了模拟研究,在这个思路的指引下对由双层银膜和位于其间的双球形银粒子组成的样品池模型在隐失波和平面波激励下所产生的近场增强效应进行了最佳条件的数值模拟研究。我们的数值模拟结果发现,在这这种金属纳米结构中不仅可以产生常规的“热点”,而且由于电子回路的形成,还产生了一种“非常规热点”。由于在专利样品池中“热点”数目的增多,据此可以在很大程度上提高拉曼探测信号的灵敏度。在文中我们还通过对样品池模型中的几个关键参数进行变化模拟,完成了优化设计。该样品池的制作和实验验证已在本课题组中基本完成。四、金属表面增强拉曼散射效应的另一个重要的应用就是镀金属膜的探针尖,其尖端的局域场增强效应使得纳米光纤探针在很多领域中可得到广泛应用,如超高密度光存储、表面修复、多光子分子的荧光光谱以及实现光镊的纳米分辨等。其中一个关键问题,那就是在探针尖端必须能够得到足够的场增强。本文对镀金属膜的全反射四棱锥形探针尖在底部照射方式下产生的近场增强分布进行了数值模拟。结果表明:这种形状的探针在其尖端不仅能获得纳米尺度的聚焦光斑,且光斑的强度也得到了显著的增强。文章最后,作为对今后模拟研究意见的初探,初步讨论了不规则形状的物体与电磁场之间的相互作用更设用的数值模拟方法一有限单元法(Finite Element Method,FEM)。FEM的单元自适应不均匀网格划分的独特优势,可使其成为今后研究不规则形状金属纳米结构-尤其是金属尖的“热点”场分布更为有效的一种数值模拟方法。更多还原

【Abstract】 With the discovery of the laser, the application of the detection of the weak signal and the application of the computer, the applications of the Raman spectrum in many research domains are developed rapidly. And some techniques, such as Fourier Transfer Raman, Resonance Raman, co-force micro-Raman, high temperature Raman, PARS, and SERS, made the sensitivity and the ratio of signal to noise of Raman spectrum improvement. Among of them, SERS which can efficiently and highly enhance Raman signal attracts more and more attentions. The high sensitivity of SERS makes its applications extension from the material, chemical, biology to the medicine.This thesis concentrates on the numerical simulations of the near-field enhancement distribution of different kinds of metallic nano-structures under the near-field excitation and aims to explain the electric enhancement in the SERS theoretically and provides scientific basis for the SERS experiment.The thesis mainly includes the following four parts:1. The frequency-dependence finite-difference time-domain formulation is extended to deal with the metallic material. With a complex frequency-dependent permittivity, the metallic material can be described by Drude model. The equivalent incident wave method which is widely used in the multi-layer medium system is used to resolve the setting problem of evanescent wave excitation arisen from the total internal reflection above the interface. Thus a complete numerical simulation method is got to deal with the interaction between the metallic nano-structures with the random shape and the electric field. And its validity is approved in this thesis.2. The near-field enhancement distribution of the metallic nano-structures with the random shape under the near-field excitation is simulated. Through the numerical simulation of the near-field distribution of the different metallic nano-structures under the near-field excitation, the dependence of the field enhancement factor on the shape of configurations is illuminated with the concept of electric free path, dielectric function, plasma frequency and so on.The key elements which are related to the Raman signal in SERS are the metallic optical properties, configuration, size, congregation, and the orientation of the metallic colloids. The near-field enhancement distributions of bi-spherical nanoparticles and the multi- spherical nanoparticles system under the excitation of the plane wave are given to explain the problems brought forward by Nie in his paper in 1997 and provide the theory basis for the experiments.3. Design of the super sensitivity near-field SERS sample cell is the key technique for extending the application areas of the SERS technique and especially for the single molecule SERS technique. Though the intense field enhancement can be got for the metallic nano-structures with more complex shape, immature producing technique limits their applications in an extensive way. Spherical metallic nanometer particles colloids are still the best choice for the field enhancement. The field distribution for different arrays of bi-spherical Ag particles under the excitation inspires us to design the sample cell composed of two layers of Ag film and Ag particles between them. The optimal design parameters are also simulated in the thesis and the validation of the sample cell has been done experimentally. Several problems of the sample cell on the experiment are pointed out and the means are put forward in this thesis.4. The other important application of SERS technique is the probe coated with metallic film, around the end of which the strong local field enhancement effect can be produced. The effect made the probe coated with metallic film extensively used from data storage, surface modification, multi-photo molecular fluorescence spectroscopy, to optical tweezing at nanometer scale resolution. A kind of pyramidal dielectric probe fully coated with thin metallic film is simulated and the results show that the structure can provide not only the nanometer spot but the greatly enhanced field at the tip of the probe.A kind of more effective numerical simulation method is introduced to deal with the interaction between the metallic nano-structures with more complex shapes and the electric field.更多还原