The Simulation Of Localized Surface Plasmon Modes Of Metal Nanoparticles

Surface plasmonology is an important part of nanophotonics.For nano-metal par-ticles,their specific surface area is much larger than that of bulk metal.The peculiar optical properties of noble metal nano-particles originate from the interaction of elec-tromagnetic fields with free electrons on the metal surface.This physical process will cause an enhanced near-field on the surface of the nano-particles.The optical proper-ties of nanoparticles with different shapes are also different.Classical electrodynamics shows that the boundary conditions are the key factors affecting the diversity of nanopar-ticles.For a smaller scale(a few nanometers)of nanoparticles,quantum effects must be considered.This paper first introduces the coupling phenomena between two types of surface plasmon polaritons and plasmon polaritons and their applications.Second,we briefly describe the experimental characterization methods and numerical simula-tion methods for surface plasmons.Finally,we introduce the quantum effects of surface plasmons.(1st chapter)In the frame of classical theory,theoretical simulations of the external electric field to excite the localized surface plasmons of nanoparticles need to solve Maxwell's equa-tions.External applied electromagnetic fields include plane wave and high energy elec-tron beam.The theoretical basis for the scattering and absorption processes between polarized light and particles is described in detail.The cross-section formulas for light scattering,absorption and extinction of particles are deduced.The theoretical basis for interaction between high-energy incident electrons and nanoparticles is also presented.The expression of the probability of energy loss of incident electrons.As for the nu-merical simulation,only the Mie theory can obtain the exact optical properties of the spherical particles.We introduce discrete dipole approximation(DDA)and boundary element method(BEM)to solve Maxwell's equations for arbitrary shape particles.For the optical properties of small-size clusters,the time-dependent density functional the-ory needs(TDDFT)to be used for modeling.(2nd chapter)The research about localized surafce plasmon modes of metal nanoparitlces have been performed in this thesis as follows:1.The peculiar optical properties of noble metal nanomaterials originate from the excitation of various localized surface plasmons,and the LSP mode strongly depends on the morphology,size,and spatial configuration of the nanoparticles.This chapter we use the DDA method to simulate the LSP mode of silver nanoparticles.Here we have developed the DDSCAT7.3 source program,and enable it to deal with excitation of localized surface plasmon by electron incident.Thus the simulated electron energy loss spectra(EELS)can be obtained.For the time being we call it DDA-EELS.Comparing with the optical attenuation spectrum calculated by the original software,we verified the feasibility of using the DDA method to simulate the EELS spectrum.Then,we used DDA-EELS to study the LSP coupling of the Ag nanosphere dimer.And the theoretical simulation results agree qualitatively with the experimental results.Furthermore,we studied various LSP modes of Ag nanocubes,and combined EELS maps with surface charge distribution to analyze the LSP modes.(3rd chapter)2.The asymmetric Ag-Ag heterodimer plasmon coupling model was simulated systematically using the boundary element method(BEM).Here we have constructed nanoparticles with different surface curvatures(adjusting the rounding parameter e)and combining them into an asymmetric Ag-Ag dimer.Combining with the simulated elec-tron energy loss spectrum and eigenmodes,the evolution of coupled surface plasmon modes is analyzed.Here,for the first time,we discover the degeneracy and degeneracy of coupled modes by adjusting the symmetry of particle morphology,and the charge dis-tributions of these modes are always non-degenerate.At the same time,we also found that the coupled gap mode G2(gap mode)can only be excited in Ag-Ag heterodimers with sufficiently small spacing.And it will be strongly influenced by the morphology of the gaps.The sleek appearance can also cause blue shift in the resonance energy of the dipole coupling mode obviously.Our simulation results show that the optical proper-ties can be effectively adjusted by different asymmetric dimers.On the other hand,the optical response of the higher-order coupling mode is less affected by the topography effect than the low-order coupling mode.In addition,we also researched the plasmon ruler for asymmetric Ag-Ag dimer,and proved that the generalized plasmon rule can be applied to predict the red shift of coupling dipole mode of asymmetric dimer,which are caused by the change in separation distance.(4th chapter)3.Hollow nanostructures provide a new way of regulating surface plasmon modes due to the presence of internal cavity surfaces.We used the boundary element method to simulate the electron energy loss spectrum of hollow nanostructrues.Here we mainly studied the multipole surface plasmon mode of the hollow silver nanoprism(HSN).Here we consider the effect of cavity size and location,and compare the local surface plasmon modes of HSN with those of a perfect silver nanodisk.With increasing the cavity size,the multi-pole resonance mode will be red-shifted.The modes A and C will have similar redshift trends and both follow the plasmon rule.We can use the dipole coupling model to give a physical explanation of this phenomenon.In addition,we also found that the degeneracy of the original model will be separated by changing the position of the cavity.And these degenerative modes will show the opposite movement tendency with the cavity position.Combined with the analysis of the intrinsic charge distribution of surface plasmons,this phenomenon rises from the different coupling nature of degenerate modes.Finally,we discussed the sensitivity of the plasmon modes of HSN to the dielectric environment.By adjusting the size and position of the cavity,we can obtain high refractive index sensitivity(RIS).(5th chapter)4.For a few tens of nanometers to a few hundred nanometers,the optical proper-ties of the metal particles can be well simulated using classical theory.However,when the size is further reduced,the atomic positions can not be ignored anymore.Thus we must simulate the excited states of the system from the perspective of quantum mechan-ics.We used time-dependent density functional theory(TDDFT)to study the optical excitation properties of small-sized metal nanoclusters.Here,we consider the optical absorption spectra of Na and Ag nanoclusters by pulsed light.The optical properties of clusters with different atomic numbers of the same material are very different.The optical properties of nanoclusters with many atoms are closed to the results from clas-sical theory.In addition,we also studied the plasmon coupling behavior of nanocluster dimer.For the smaller dimer spacing,we can find a weak tunneling phenomenon and the optical absorption spectrum intensity is significantly reduced.(6th chapter)...