金属纳米二聚体和三聚体的等离激元耦合研究

【作者】 张静；

【导师】 李永放；

【作者基本信息】 陕西师范大学 ， 光学， 2020， 博士

【摘要】 近些年,人们已经设计出各种金属纳米二聚体或者三聚体结构。在外部光场的作用下,由环、棒或三角形等微观纳米结构组成的二聚体或者三聚体可以产生类似于原子系统中的电磁感应透明(Electromagnetic Induced Transparency,EIT)现象,被称为等离激元诱导透明(Plasmons Induced Transparency,PIT)。事实上,PIT效应可以说是类-EIT效应。由于PIT效应在慢光,等离激元传感和低损耗超材料领域具有潜在的应用价值,因此人们设计了各种纳米复合物结构用于研究PIT现象。Fano效应和类电磁诱导透明效应(EIT)已成为研究金属纳米元件之间相互作用的热点问题。无论是Fano效应还是EIT效应,等离激元的耦合过程都在其中起着重要的作用。尽管对于不同形状和结构的纳米粒子二聚体或者三聚体可以产生各种物理现象,但是等离激元之间的耦合过程是产生这些现象的主要物理原因。本文主要研究内容如下:(1)文中基于有限元方法,数值计算了在外加光场的作用下,两个非对称空心纳米环组成的二聚体的吸收光谱、电荷和电场的分布特征。提出了一种新的(Extended Coupled Lorentz Oscillator Model)扩展耦合洛伦兹振子模型(简称 ECLO模型),与传统的(Coupled Lorentz Oscillator Model)耦合洛伦兹振子模型(简称CLO模型)进行对比,该模型中加入了耦合相位因子,用以描述二聚体中类似电磁诱导透明(EIT)的现象中两个吸收峰表现出的不对称行为。利用ECLO模型揭示了光谱特征和类EIT效应的物理机制。研究发现,干涉相消效应在类EIT效应的产生中发挥着关键作用;同时耦合相位因子会对两个吸收峰衰变起到调制作用。由此通过物理模型的理论分析对数值计算结果给出了很好的物理解释。(2)提出了一个新的(Bright-Dark Mode Coupling Model of Plasmons)等离激元中的明-暗耦合模型(简称BDMC模型)。耦合模型包含库仑势和静电势;描述了二聚体中等离激元之间的相互作用关系,以及纳米单元之间距离的变化对耦合的影响。定义了一个新的物理量:等离激元共振频率偏移量,用于描述等离激元发生耦合时频率的变化量。在BDMC模型中库仑势和静电势共同决定着频率偏移和耦合系数的变化特征。利用BDMC模型不仅解释了随纳米棒间距变化的吸收光谱峰值的不对称行为,而且可以得到等离激元之间的共振频率差。同时利用BDMC模型所获得的参数,并利用上面的(ECLO)模型对吸收谱进行了模拟,理论模型计算结果与数值模拟结果相一致。(3)研究了由一个水平和一对竖直的金(银)纳米棒组成的三聚体在光场的作用下的吸收光谱特征;研究了水平棒和两个竖直棒产生的等离激元之间发生的耦合作用。通过对纳米结构对称性的破坏和结构参数的改变,研究了吸收光谱的分布变化,观测到了 PIT现象。结构的变化,特别是对称性的变化可对PIT进行动态调谐。文中利用三个谐振子耦合模型模拟研究了纳米棒三聚体之间等离激元的耦合问题,研究结果对数值模拟给出物理解释。总之,论文主要是研究了等离激元中的各种耦合问题,针对三种耦合中的物理现象提出了相关物理模型,解释了所设计的金属纳米结构中的二聚体和三聚体的各种吸收光谱所展示的物理特性。这些研究结果为今后设计纳米级别的人造原子、分子和光开关以及慢光应用和增强非线性效应提供了理论参考。而且,扩展我们的理论来研究纳米等离激元中的其他相关物理问题也是非常有趣的。更多还原

【Abstract】 In recent years,various metal nanometer dimer or trimer structures have been designed.Under the action of external light field,dimers or trimers composed of the rings,the rods or the triangles can produce similar to the electromagnetic induced transparency(EIT)phenomenon in atomic systems,which is known as plasmons-induced transparency(PIT).In fact,the PIT effect is the like-EIT effect.Due to the potential application value of PIT effect in the fields of slow light,plasmons sensing and low loss metamaterials,the various nanocomposite structures have been designed to study the PIT phenomenon.Fano effect and EIT effect have become hot topics in the study of the interactions between metal nanomaterials.Both the Fano effect and the EIT effect,the coupling process of plasmons plays an important role.The coupling processes between the plasmons are the main physical cause of these phenomena although a variety of physical phenomena can be generated for dimers or trimers of different shapes and structures.The main contents of this paper are as follows:(1)Based on the finite element method,the distribution characteristics of the absorption spectra,the charge and the electric field of two asymmetric hollow nanocircles under the action of external light field are investigated.A new extended coupled Lorentz oscillator model(ECLO for short)is proposed,which is compared with the traditional coupled Lorentz oscillator model(CLO for short),in which the coupled phase factor is added to describe the asymmetric behavior of the two absorption peaks in the like-EIT phenomenon in the dimer.The ECLO model is used to reveal the spectral characteristics and the physical mechanism of like-EIT effects.It is found that the destructive interference effect plays a key role in the generation of like-EIT effect.At the same time,the coupling phase factor modulates the decay of the two absorption peaks;the theoretical analysis is agreement with the numerical results.(2)A new bright-dark mode coupling model of plasmons(BDMC for short)is proposed.The coupling model includes the Coulomb potential and the electrostatic potential.This model describes the interaction between the plasmons and the effect of the distance variation between nano-elements on the coupling.A new physical quantity,i.e.,the resonant frequency offset of plasmons,which describes the change of resonant frequency when the coupling of the plasmons occurs.In the BDMC model,the variation characteristics of the frequency offset and coupling coefficient are determined by the coulomb potential and the electrostatic potential.The BDMC model explains not only the asymmetric behavior of the absorption peaks with the change of the nanorods gaps,but also the resonant frequency difference between the plasmons is obtained.At the same time,the absorption spectra are simulated with the ECLO model,in which the parameters are obtained by the BDMC model,the calculated results are agreement with the numerical simulation results.(3)The absorption spectral characteristics of a trimer,which composed of a horizontal and a pair of vertical gold(silver)nanorods under the action of light field,are investigated;The coupling process between plasmons is explored.By destroying the symmetry of the nanostructure and changing the structure parameters,the distribution of the absorption spectra are studied,and the PIT phenomenon is observed The dynamic tuning of PIT can be made by the change of structure,especially the change of symmetry.A three harmonic oscillator coupling model is to simulate the coupling problem between plasmons in the trimer.The results obtained with the three harmonic oscillator coupling model give a physical explanation for the numerical simulation.In a word,this paper mainly studies various coupling problems in plasmons,the physical models for three coupling physical phenomena are proposed,and the physical properties are explained of dimer and trimer absorption spectra in the designed metal nanostructures with these physical models.These results provide a theoretical reference for the design of the nanoscale artificial atoms,molecules and optical switches,as well as slow light applications and enhancement of nonlinear effects.Furthermore,it is interesting to extend our theory to study other related physical problems in the plasmons.更多还原