Abstract
Gold nanostructures with optical properties of localized surface plasmon resonance (LSPR), which is related to strong light absorption and scattering and local electromagnetic field enhancement, have been applied in chemical and biological sensing owing to their high stability. To analyze the hybridization of LSPR modes, which are modified by symmetry breaking and a substrate, complicated fabrication processes have been reported in the literature. In this thesis, substrate-mediated surface plasmon resonance modes of three types of gold nanostructures including nanoparticles, single nanowires and thin film patterns on dielectric substrates are investigated by dark-field optical microscopy and finite-difference time-domain (FDTD) simulation. These three nanostructures are created by relatively simple fabrication techniques. In a spectrum of gold nanoparticles with spherical symmetry on a dielectric substrate, distinct resonance peak shifts and quadrupolar resonance peaks resulting from the degeneracy and hybridization of LSPR modes can be observed. Moreover, the system can be served as a model for LSPR hybridizations. For single gold nanowires, which are fabricated by atomic force microscopic lithography, the substrate-mediated hybridization is a significant factor in LSPR modes. In a scattering spectrum, the peak at 621 nm in wavelength is resulting from the superposition and substrate-mediated hybridization of two LSPR modes excited upon the polarized electric field. On the other hand, the peak at 485 nm is related to the thickness of the nanowire. Furthermore, a slightly geometric change of the structure close to the substrate surface might provide observable shifts of LSPR modes, that explains the importance of the substrate-mediated hybridization. The plasmon modes observed at the edges of gold thin film patterns are thoroughly adjusted by the existence of the induced image charge on the dielectric surface. In a scattering spectrum, the main peak at 645 nm is due to the superposition of plasmon modes with similar substrate-induced electric field. In addition, minor peaks at 588 nm and 507 nm are attributed to the substrate-mediated plasmon modes from the vertical and the bottom parts of edge structures, respectively. The above results reveal that the substrate-mediated hybridization is enhanced with a larger contact area between nanostructures and the dielectric substrate.