【作者】 王刚；

【导师】 崔一平；

【作者基本信息】 东南大学 ， 物理电子学， 2005， 博士

【摘要】 金属纳米粒子因其突出的光功能特性,很可能成为未来光电子器件的基础材料之一,其极大的非线性极化系数预示着这类材料有丰富的光功能现象和极大的应用潜力。对其中的物理机制进行研究并加以利用,从而改善或有目的地控制它的非线性无疑是十分重要的。通过研究,人们认识到金属纳米粒子的线性光学性质与粒子表面等离子体共振（SPR）过程密切相关。SPR是纯粹的纳米效应,对边界条件特别敏感,这决定了金属纳米粒子的性质具有强烈的尺寸和形状依赖性。相对于线性光学性质,对金属纳米粒子非线性光学性质的研究相对欠缺。本文以银纳米粒子为对象,研究了聚集对其二阶和三阶非线性光学性质的影响,取得了一些成果,主要内容包括以下几个方面。根据实验室条件建立了超瑞利散射（HRS）检测系统。HRS实验光路相对简单,但是HRS信号容易受到其它因素的影响,因此对实验条件的要求比较苛刻。我们在实验中采取了离焦激发方式,有效地避免了其它非线性过程（如受激布里渊散射、介质击穿等）对HRS信号的干扰。用标准样品（pNA）,从信号光强与溶液浓度的关系、信号光强与入射光强的关系和信号光谱三个方面仔细地对建立的HRS检测系统进行了校准和检查,证实此HRS装置各单元工作状况稳定、光路配置合理、测量结果准确。采用改进的Lee和Meisel方法,制备了粒径为10.5 nm且分散高度均匀的银纳米粒子。运用HRS检测系统,采取内参法确定了银粒子的二阶非线性极化系数,并对其二阶非线性极化过程进行分析。由KNO3诱导聚集,通过粒子尺寸测量和透射电镜（TEM）观察,表明聚集方式为链状聚集。文中分析了聚集效应对银纳米粒子吸收光谱的影响。运用HRS技术,研究了银纳米粒子聚集体的二阶非线性光学响应。实验结果表明,银粒子聚集体的HRS增强效应存在最佳聚集尺度,当聚集体平均尺寸为120纳米时,HRS信号强度比聚集前增大了约15倍。经过分析发现,聚集导致了银纳米粒子表面局域场强度的增强和分布的改变,并通过表面和体贡献机制对二阶非线性极化过程产生影响。在用柠檬酸修饰的银纳米粒子中加入吡啶后,吡啶中的氮原子与羟基氢形成氢键（-N...H-O-）,无氮原子的另一端疏水,在表面张力的作用下,也会引起银纳米粒子的链状聚集,并且这种聚集体与由库仑力引起的聚集体也必然存在不同。文中对此种聚集体的吸收光谱和二阶非线性光学性质进行了实验研究,从实验上观察到纯粹局域场增强对HRS信号的影响。实验结果表明,此种聚集体同样存在最佳聚集尺度,当聚集体平均尺寸为120纳米时,HRS信号强度比聚集前增大了约5.5倍。分析指出,聚集体中银纳米粒子的间距增大导致粒子表面局域场强度增强的减弱,所以此种聚集体有较小的HRS增强效应。文中研究了聚集效应对三种不同粒径银纳米粒子二阶光学非线性的影响。实验结果表明,三种尺寸的银粒子聚集体都存在最佳聚集尺度,使HRS响应最大;随着粒径的增加, HRS信号的增加倍数下降。我们将其中的原因总结为表面效应和相位延迟效应共同作用的结果。文中建立了偶极叠加模型和偶极场相互作用模型,进行了理论推导。运用理论推导结果,对上述实验现象进行了模拟计算,取得了较一致的结果。建立了一套较好的Z-扫描实验系统。运用Z-扫描装置,首次报道了聚集对银纳米粒子非线性折射率的影响。结果表明:聚集使银纳米粒子溶液的非线性折射响应增大了约4倍,局域场增强是其中的主要因素。实验中也观察到最佳聚集尺度的现象。运用理论分析计算和Z-扫描技术,从局域场角度研究了不同表面修饰的Fe₂O₃纳米粒子的非线性折射率。理论计算结果与实验现象吻合的较好。更多还原

【Abstract】 Noble metal nanoparticles have attracted significant attention because of their good chemical stability, unusual optical and electronic properties, and their potential applications in optoelectronic and photonic technologies. Their linear optical characteristics are recognized as being dominated by the surface plasmon resonance （SPR）, which is sensitive to the boundary conditions. So, the shapes and sizes of these particles would provide important control over their linear and nonlinear responses. Parallel to the linear optical properties, their nonlinear optical properties have been reported on fewer occasions. In this dissertation, the aggregate influences on the second-order and third-order optical nonlinearities of silver nanoparticles have been studied. The dissertation is mainly comprised with the following parts.The Hyper-Rayleigh scattering experimental system has been established. The optical route of HRS is relatively comprehensible, but the HRS signal is easily disturbed. Therefore, the experimental condition is restricted. In practice, the excited way of off-focus is applied to avoid some unwanted nonlinear processes, such as stimulated Raman scattering, stimulated Brillouin scattering and dielectric breakdown etc. By using pNA, the quadratic dependence of the HRS signal with the incident light intensity, the linear dependence of the HRS signal with the material concentration, and scattering intensity as a function of the wavelength around 532 nm were checked to access the HRS system.A highly monodisperse 10.5 nm diameter silver nanoparticle has been synthesized by following the Lee and Meisel method. According to the inner reference method （IRM）, the particle’sβis determined by HRS. The second-order polarization process has been discussed.The HRS has been employed to determine the second-order nonlinear optical response from the silver nanoparticle and its aggregates, which are induced by the presence of KNO₃. Through size distribution and TEM measurements, the morphology of aggregated silver nanoparticles has been observed as a chain-like aggregation. The HRS results reveal that there is an optimum size for the aggregates to yield the maximum HRS response. The dramatically enhanced second-order nonlinearity was explained by the enhanced electromagnetic （EM） field near the surface of thesilver nanoparticles as they approaching. The enhanced local EM field participates to the nonlinear polarization through surface and bulk contributions.The silver aggregates could also be caused by the addition of pyridine, and these two kind aggregates are obviously different. The different enhancement between the aggregates was considered with the diversity of separated distance between silver nanoparticles.The HRS signals from three volume silver nanoparticles with aggregate effects have been measured. With the size increase, the HRS enhancement is decreased. The observed size dependence of the second harmonic generation from the aggregates is explained by assuming the surface contribution and phase delay effect.The models of induced dipolar accumulation and interaction among the dipolar fields are established to analysis the above phenomena, and good agreements have been found between experimental results and theoretical simulations.The aggregate influence on the nonlinear refraction of silver nanoparticles has been observed by Z-scan technique. The nonlinear refraction of the silver aggregates is enhanced by 4 times at the optimum size of 120 nm. This phenomenon is explained by the local field effect.Using the Z-scan technique and theoretical calculation, the local field effect on the nonlinear refraction of Fe₂O₃ nanoparticles has been studied, and a consistence is reached.更多还原