Surface Plasmon Coupling And Optical Chirality Of Metal Nanostructures

When the metallic nanostructure is excited by incident light,its free electrons will interact with incident light.It will form near field electromagnetic wave propagating along the metal surface,which is called surface plasmon(SP).Based on the boundary constraints of metallic nanostructure,SP can be divided into localized surface plasmon(LSP)and surface plasmon polariton(SPP).Due to surface plasmon resonance,metallic nanostructure demonstrates unique optical features,which have attracted wide attention of researchers.These metallic nanostructures have been successfully applied to biosensor,medical diagnosis,photoelectric functional materials,ultra-high density of optical storage and new energy fields.In this dissertation,several kinds of metallic nanostructure are proposed and designed.They are simulated with finite element method(FEM),and studied in theory via surface plasmon resonance coupling,surface plasmon induced transparency(PIT)and optical chirality.The main research work of this dissertation is listed as follows,1.The plasmonic resonance of double sector metallic nanostructure is studied with theoretical analysis and numerical simulation method.The results show that localized electromagnetic(EM)fields can be significantly enhanced at the tips of the structure and the gap between the two sectors.In this nanostructure,multiple resonances are produced,and directional far-field energy radiation can be observed.It is found that resonant peaks are generated by coupling of bonding mode of the two sectors,based on the analysis of charge and electric field distribution.Besides,the position of resonant peak can be adjusted by tuning structural parameters,such as central angle,included angle and the distance of the two sectors.The research results can provide reliable theoretical evidence for the development of optical nanoantennas and biological spectrum sensing2.A composite metallic nanostructure is proposed in this dissertation,composed of.a silver nanoring and a built-in nanocross(NRC).The plasmonic resonance is studied when nanoring and nanocross are connected or separated respectively.When the nanoring and nanocross are connected,LSP peak can be modulated and ultrahigh-order plasmon resonance peaks and electric field enhancement can be monitored via tuning structural parameters.The parameters include intersection angle,intersection position,and the size of the structure.Especially when intersection angle of nanocross is 90°,the nanostructure is polarization-independent.In addition,characteristics of extinction spectra are analyzed with mode coupling mechanism,via observing the electric field and charge density distribution.In comparison,when nanoring and nanocross are separated,new high order resonance mode is generated via changing distance between nanoring and nanocross.In summary,this study provides potential ideas for designing tunable filters.3.Surface plasmon induced transparency(PIT)effect is obtained via designing two-dimensional and three-dimensional metallic nanostructures,which are composed of nanorings and nanorods.The numerical results show that PIT resonances can be tailored by structural arrangement,spatial symmetry and environmental properties of surrounding medium.By adjusting the relative position of the nanorod in the nanoring,the symmetry of the structure is broken,and two PIT windows appear simultaneously on the transmission spectrum.It is deduced that the transparent window at the short wavelength is generated by bright-dark mode coupling,while the one at the long wavelength is generated by bright-bright mode hybrid coupling.Furthermore,PIT of bright-dark coupling is analyzed in details,based on the Lorentz oscillator model.Therefore,PIT phenomenon obtained by two kinds of different model couplings can provide a good reference for new spectral filter.4.Based on the Born-kuhn model,the study is done for the surface plasmon resonance and circular dichroism of double layer rotation configuration.The results show that there are strong circular dichroism(CD)and optical chirality for this structure.It depends on the parameters of the structure,such as the size of the structure and the spacing between the upper and lower layers.In theory,the upper and lower nanostructures are equivalent to electric dipoles,based on the Born-kuhn model.By analyzing the coupling mode of electric charge in the upper and lower layer,the mechanism of circular dichroism and the shift of the CD resonance are revealed.Therefore,the near-field enhancement effect and chiral optical properties of the designed chiral nanostructures can provide experimental basis for biological targeted molecular detection and spectral sensing.