Optical Resonance Characteristics Of Plasmonic Core-shell Nanostructures

Surface plasma resonance is the collective oscillation of free electrons in noble metal when the excitation frequency of the incoming light field matches with metal particle surface free electron’s eigen frequency. Surface plasma resonance has many interesting properties, such as to be able to produce significant local field enhancement and breakthrough light diffraction limit. Plasmonic core-shell nanostructure is a kind of typical symmetric structure, supporting many interesting resonance phenomena and has wide practical applications. Plasmonic core-shell spheres can be used in high-resolution optical imaging, surface enhanced spectroscopy, optical detection and biological sensing, etc. On the basis of the simple spherical core-shell nanostructure, we study the optical resonance properties of multilayered spherical core-shell nanostructures, as well as imperfect spherical core-shell nanostructures. In particular, the possibility of using them as optical taggsas discussed.This thesis mainly use the Mie scattering theory and the finite integral method(FIT) to study the optical resonance characteristics of spherical core-shell nanostructures, both of them give comparable results. We focus on the optical scattering cross sections under the action of light field and the near field distribution. For the imperfect spherical core-shell structure, we mainly employ FIT method because the Mie theory fails in such cases.It is found that there is an obvious Fano resonance in single-layer spherical core-shell structure. The resnance frequency shifts as the radius ratio of varies. Increasing the surrounding medium dielectric constant, the scattering cross section peaks "red shift" phenomenon, while increasing the dielectric constant of core medium, the resonance peak position remains unchanged. In two-layer core-shell nanostructrues, by changing one of the aspect ratio, one can switch from a resonance state and cloaking state alternately, this is similar to eletromangetically the induced transparency phenomena.Very intetestingly, four layers spherical core-shell nanostructures support Fano-comb scattering spectra. By change the dielectric constant of the surrounding medium and the core, the Fano-comb scattering spectrum is nearly unaffected. This is mainly due to the geometric topology. For modeling several imperfect spherical core-shell nanostructures, such as nonconcentric and rough suface ones, we found that the Fano-comb scattering feature remain robust. This is also due to the geometry core-shell topology of the nanostructure.Our results provide theoretical guidience for defining core-shell nanostructures, in terms of controlling surface plasmons resonances. They also present possibility for surface enhanced spectroscopy application base on local surface plasmons resonance, such as optical tagging.