Plasmon Coupling Studies Of Metal Nanodimers And Trimers

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.