Surface Plasmon Polaritons Coupling Characteristics Of Silver Nanoparticle Pairs Array On The Thin Silver Film

Surface plasmon polaritons (SPPs) are collective oscillation of free electrons on metallic surface interacted with incident light. The intense localized electromagnetic field of SPPs can break diffraction limitation realizing optical transfer and modification in sub-wavelength. In addition, SPPs has wide application and development potential, such as surface enhanced spectroscopy, biochemistry nanosensing and enhancing nonlinear optical signals. As an attractive nanostructure, metal nanoparticle pairs array on thin film structure has distinguishing characteristics. Nanoparticle pairs exhibits a very intense field enhancement at the junction region, known as “Hot Spot”, resulting from localized surface plasmon (LSP). Moreover, wave vectors matching is satisfied by periodic lattice which raise the possibility to excite and manipulate SPPs on the thin film. This kind of metal nanostructure is called LSP/LSP/SPPs (LLS). In this thesis, the plasmonic coupling between LSP from silver nanoparticle pairs and SPPs from thin silver film has been studied in details.Nonlinear polarizability of silver nanocluster homodimer and heterodimer has been studied within density functional theory. The hyperpolarizabilities of silver nanocluster dimer are sensitive to various separations of the two nanoclusters. Different from classical theoretical studies, there exists the maximal hyperpolarizability defined by an appropriate separation, and the longitudinal tensor component of nanocluster dimer is significantly larger than that of the sum of two single silver nanocluster monomers. The enhanced nonlinear polarizability of homodimer is more susceptible to the crucial transitions between highest occupied molecular orbitals and the lowest unoccupied molecular orbital (HOMOs-LUMO). The enhanced nonlinear polarizability of heterodimer results from the charge transfer between two nanoclusters giving rise to the enhanced nonlinear optical coefficient.The multiple SPPs excitation of a two-dimensional periodic array of silver nanoparticle pairs on a thin silver film has been studied with multiple-scattering formalism. The excited multiple plasmonic modes, including SPPair and SPPqua on the interface of thin silver film-air and film-quartz, reveal that the dispersion relationship of surface plasmon polaritons on metallic film is modified by doubly periodic lattice due to the wave vectors matching conditions are satisfied. Moreover, the plasmonic modes are directly controlled by the geometry parameter. Based on thickness of silver film, we realize the resonant energy transfer between LSP and SPPs. Based on the gap between nanoparticle pairs array and silver film, we also realize the resonant mode transition between SPPair and SPPqua.The optical absorption spectrum of a periodic array of silver nanoparticle pairs on a thin silver film has been investigated with multiple-scattering formalism. Surface plasmon polaritons on thin silver film is excited and enhanced by about four times compared with that from monomer array. This enhancement results from the coupling between the two nanoparticles’ plasmonic coupling of symmetry mode and anti-symmetry mode. The distance-dependent nanoparticle plasmonic coupling modes. Moreover, the distance in horizontal direction also effects on the plasmonic coupling modes from nanoparticle pairs. In an appropriate excitation band, the nanostructure realizes energy trapping and directional energy transmission.A thin silver film is attached to a periodic array of asymmetry nanoparticle pairs consisting of two vertically spaced silver nanoparticles of different radii. The plasmonic coupling between LSP and SPPs of the metallic slab has been investigated. Depending on the radius of nanoparticles, asymmetry nanoparticle pairs array presents a simple nanoscale geometry which gives rise to remarkable plasmonic properties of electric dipolar resonance and elelctronic quadrupolar resonance. Due to the coherent interference of the localized nanoparticle plasmons (discrete mode) and surface plasmon polaritons of metallic film (continuous mode), the reflection spectrum represents a sharp asymmetric Fano resonance dip, which is strongly sensitive to the refractive index of the surrounding embedded dielectric host. The physical features contribute to a highly efficient plasmonic sensor for refractive index sensing with sensitivity of~1.510-3RIU/nm.