A Study on Surface Plasmon Properties in Metallic Nanostructures
Surface plasmon (SP) excitations strongly influence the optical properties of metal nanostructures and have received great interest for nano-optics applications. This thesis contributes to the understanding of light-SP interactions on various SP-supporting systems, including metal nanoparticles cluster networks, planar patterned metal films, and dye-doped metal nanorods.
Optical and electrical measurements on the noble metal nanoparticle networks demonstrate the dominant role of the inter-particle coupling through resonant surface plasmon excitations in these samples. A cluster model was developed to evaluate the effective dielectric function and the electrical conductivities of the nanoparticle networks by taking into account the effects of SP excitation on noninteracting nanoparticles and the coherent coupling among neighboring particles. A percolation threshold of 0.18 was predicted in the as-fabricated samples, consistent with the conductivity measurement. Simulations on the local field distribution for nanoparticles in small aggregates like chains, circular rings, were conducted to study the aggregation effects on the near-field enhancement factors.
Furthermore, time-resolved spectroscopy has been employed to study SP-induced temporal delays of femosecond pulses through aluminum films perforated with subwavelength hole arrays. The 60fs and 100fs delays were measured through the single- and double-layered samples in resonant with the major SP modes. A surface-plasmon-mediated transmission model is proposed to explain the dynamics of the transmitted pulses and the SP-photon coupling mechanism via the nanostructures.
The final part of the thesis investigated the plasmon-enhanced fluorescence from hybrid nanostructures of Au nanorods coated with fluorophore-doped silica shells. Due to the excitation of the localized surface plasmon, the excitation coefficient and the emission rate of the dye molecules increased around the longitudinal SP mode of the Au nanorods. The quantum yield of the structure was estimated to increase from 0.31 to 0.56 with the present of SP excitation.