表面等离激元金属纳米结构中空间非局域效应的实验与理论研究

At subwavelength scale, many experiments have demonstrated that both far- and near-field plasmonic properties of metallic nanostructures can be effectively described by classical electromagnetics. Rapid development in nanofabrication and characterization techniques has driven the miniaturization of plasmonic metal nanostructures towards nanometer and even sub-nanometer feature sizes. At such small scales, the electron-electron interactions induced by classical Coulomb force and quantum Pauli repulsion have significant effects on the macroscopic dielectric response of metals, which makes the local electromagnetic model based on free electron response fail. In this project, we will investigate both experimentally and theoretically the effects of spatial nonlocality, emerging at sub-nanomater scales, on the excitation behaviors of surface plasmons. In theory, a hydrodynamic model, which takes into account of the spatial dispersion effects of electrons in metal nanostructures, will be adopted to correct the conventional Drude model. This will be combined with the frequency-domain Maxwell's equations to obtain the coupling equations that can be used to calculate the plasmon resonance wavelength and the near-field enhancement. In experiment, layer-by-layer self-assembly, self-assembly monolayer and graphene multilayer deposition methods will be used respectively to achieve precise control over the gap distances in metal nanoparticle dimers as well as the particle-film distances in film-coupled nanoparticles. Subsequently, single-particle dark-field scattering and surface-enhnaced Raman scattering spectroscopy techniques will be used to in-situ characterize respectively the surface plasmon resonance position and the near-field enhancement factor as a function of gap distance in the two plasmonic systems. Finally, the experimental results will be compared with theory in order to reveal the critical dimension at which the spatial nonlocality must be considered in all nanophotonic systems.

在亚波长尺度下，大量实验证明经典电磁学理论可以有效地描述金属纳米结构中等离激元激发的远场和近场光学性质。飞速发展的纳米制备与表征技术已经使得等离激元金属纳米结构与器件具有了纳米甚至亚纳米级别的特征尺寸。在这种尺度下，金属中电子与电子之间的相互作用显著影响其宏观介电响应，导致传统的基于自由电子响应的局域电磁场模型失效。本项目将从理论和实验两方面探究在亚纳米尺度下空间非局域效应对金属纳米结构中表面等离激元激发行为的影响。在理论上，考虑到空间色散效应的电子流体动力学模型将被用来修正传统的Drude模型，并与频域下的麦克斯韦方程结合得到耦合方程用以预测等离激元共振峰位置和近场增强因子。在实验上，层层组装，自组装以及石墨烯多层沉积技术将被用来精确控制金属纳米颗粒二聚体的间距和纳米颗粒与金属薄膜的距离；单颗粒暗场散射和表面增强拉曼光谱术将被用来原位表征等离激元共振峰的位置和近场增强，并与理论预测比较。

当金属纳米结构具有亚波长特征尺寸时，等离激元激发的远场和近场光学性质已经不能用经典电磁学理论来准确描述，需要考虑多种量子力学效应，包括金属导带电子的空间色散效应、电子隧穿效应和电子输运等。尽管不少新近的实验和理论研究已经表明这些量子力学效应出现的迹象，纳米光子学研究领域依然缺乏一个结合理论与实验的系统研究来清晰地阐明这些量子力学效应在亚纳米金属耦合结构中的物理体现以及起作用的尺度范围。在这样的研究背景下，本项目开展了以下四方面的研究内容：（1）理论分析在亚纳米尺度下空间非局域效应、量子隧穿效应和量子电子输运等量子力学效应对耦合金属纳米结构表面等离激元共振近场增强和远场的限制效应；（2）将考虑以上三种量子效应的量子修正与经典电磁模拟结合起来，系统研究三种耦合金属纳米结构的表面等离激元远场共振频率和近场增强因子随着特征尺寸的演化；（3）采用自上而下和自下而上的方法在亚纳米尺度上精确控制三种耦合金属纳米结构的间距以及高精度的亚纳米尺寸表征；（4）结合三种原位测试手段（包括电子能量损失谱、单颗粒暗场散射光谱和共聚焦拉曼光谱技术）在实验上系统表征单个耦合金属纳米结构的表面等离激元共振频率和近场增强。通过以上研究内容的开展，系统比较理论与实验的结果，我们在物理上澄清了亚纳米尺度下空间非局域效应、电子隧穿效应和电子输运现象在三种耦合金属纳米结构中的各自主导尺寸范围，以及它们对表面等离激元共振近场增强和远场的限制效应和潜在的实际应用。