The Design and Spectral Behavior of Strong Coupling Surface Plasmon Metal Nanostructures
Author:Zhu Xu Peng
Supervisor:liu quan hui duan hui gao
Keyword:Asymmetrical breakage，Dielectric substrate，Fano Resonance，Metal micro-nanostructures，plasmon-induced transparency，Strong coupling effect，Structure orientation factor，Surface plasmon polaritons，THz wave modulation
The fascinating optical properties of noble metal nanostructures have attracted widespread attention since ancient times.These unique optical behaviors are closely related to the shapes,sizes,composition,and surrounding dielectric environment of metal micro-nanostructures.The fundamental source is the collective oscillation behavior of free electrons（surface plasmon polaritons）on the metal surface excited by an external light field,which makes the noble metal micro-nano structures exhibit a strong selective absorption and scattering.Besides,the surface plasmon resonance frequency is strongly influenced by the near-field coupling of the plasmonic nanostructures.These strong coupling effects make metal micro-nanostructures own a huge surface local and near-field enhancement capabilities under a resonant state,which can be used in highly sensitive sensing,enhanced spectroscopy,metamaterials,energy,life sciences,super-resolution imaging,micro-nano photonic device integration and other related fields.With the development and demand of high and new technology,designing metal surface plasmon micro-nanostructure systems with strong coupling effects and clarifying the physical properties which dominate the unique optical behavior is an important research hotspot in the field of surface plasmon photonics.The study aims to understand the intrinsic coupling rules between surface plasmon modes from theoretical models,develop and design strong coupled metal surface plasmon systems with extremely small features（tips,connections,and gaps）based on these intrinsic rules,achieve the high-focus of near field,realize the high-precision multi-dimensional control of resonance peak intensity,electric field polarization and spatial distribution,broaden the application range of surface plasmon strong coupling systems,and deepen the understanding depth of strong coupling effects.The main research results are summarized as follows:（1）Firstly,we carried out a systematic and in-depth study on the plasmonic coupling of an asymmetric gold disk/sector dimer,and investigated the light-matter interaction in such an asymmetric coupled complex nanostructures.The results demonstrated that the positions and the intensity of plasmon resonance peak as well as the spatial distribution of electric fields around the surface in the coupled disk/sector dimer can be tuned by changing the azimuth angle of the gold sector.Based on Simpson-Peterson approximation,we proposed a model to understand the obtained plasmon properties of asymmetric coupled disk/sector dimers by introducing an offset parameter between the geometry center and dipole center of the sector.The experimental results agree well with the simulations.Our study provides an insight to tune the plasmon coupling behavior via adjusting the plasmon dipole center position in coupling systems.（2）At the same time,we have carried out a systematic study on the optical properties of gold nano-octahedra via numerical simulations.We obtained broadening scattering spectra dominated by a hybridized bonding mode originated from the interaction between intrinsic plasmon modes when increasing the size of gold nano-octahedra in vacuum.Once placing the nano-octahedra on a high refractive index dielectric substrate,a Fano dip can be induced in the scattering spectra due to the mutual interference between a dipolar mode and a quadrupolar mode.Moreover,we found that the interference become stronger by increasing the refractive index in substrate,and the broadening scattering spectra split into two hybridized modes in a single gold nano-octahedron:the anti-bonding mode and the bonding mode.Our results for the first time give a clear description of the complex plasmonic properties of a gold nano-octahedron and provide insights in designing appropriate nano-octahedral structures for applications.（3）Plasmonic nanostructures with strong Fano resonance are of fundamental interest.Here,our systematic simulations show that rational positioning of a silver plasmonic heptamer above a highly reflective substrate mirror can significantly enhance its intrinsic Fano-resonance intensity.The silver nanodisk heptamer positioned at an appropriate distance above the reflective substrate enables 2.4 times field enhancement and 3.6 times deeper Fano-dip respectively compared to the heptamer directly placed on silicon oxide substrate.Besides,our results indicate that the Fano-dip position does not shift when the silver nanodisk heptamer gradually shifts away from the reflective substrate mirror（?60 nm）（4）In addition,we have designed a set of complex structure（consist of two gold micro-strips and two square split-ring）arrays in the THz range.The response characteristic spectrum of the THz array can be further modulated strong near-field coupling interactions due to the breaking of the structural symmetry.The experimental results were consistent with the numerical simulation results.Numerical simulations show that the structure has a strong dependence on incident polarization.Under the x-polarization,the transmission response spectrum of THz asymmetric structure array shows an obvious Fano asymmetric line-shape and plasmon-induced transparency due to the destructive coherence between the local surface plasmon resonances induced by asymmetric breakdown.Finally,we systematically studied the response of these two spectral characteristics with the asymmetric parameter s2=500 nm to the refractive index changes of the surrounding dielectric environment.The results show that the spectral features have a high quality factor Q(（69）（94）=41.64,=52.27),a high sensitivity to refractive index changes(（69）（94）=153 GHz/RIU,=236 GHz/RIU)and a large sensing figure of merit(（69）（94）=8.67,=10.1).Therefore,our designed structure arrays have a great application prospect in the field of THz refractive index sensing.