Plasmonic Properties of Gold Nanorod-based Oligomers and Arrays.

摘要

Gold nanorods are preferred candidates for future plasmonic applications due to their synthetically adjustable plasmon energies and polarization-sensitive optical responses. By assembling Au nanorods into oligomers and arrays, we can greatly tune their plasmonic features, obtain more robust and reliable plasmonic structures, and therefore greatly extend their applications. In this thesis, I report on my studies on the plasmon coupling in Au nanorod oligomers, the use of graphene to mediate the plasmon coupling between Au nanocrystals, the comparison of plasmonic properties between lithographically-written and chemically-grown Au nanorods, and the fabrication of macro-scale colloidal Au nanorod arrays and their applications as refractive index sensors.;First, the plasmonic properties of Au nanorods can be tailored in a wide range by coupling them with other Au nanocrystals. The unique spectral characteristics, together with the ability to concentrate light, of coupled Au nanorod structures endow them with great potentials in applications ranging from biosensing and optics to metamaterials. To fully realize these potentials requires a thorough understanding of the plasmon coupling behavior. In this regard, our group has investigated the coupling in Au nanocube oligomers and Au nanorod homodimers. In this thesis, I have further performed systematic studies on the plasmon coupling in heterodimers composed of two different Au nanorods or one Au nanorod and one Au nanosphere.;Extensive simulations were performed to unravel the plasmon coupling in heterodimers of nanorods aligned along their length axes. The effects of the gap distance, plasmon energy, and nanorod end shape on the plasmon coupling were ascertained. I found that the plasmon coupling between two arbitrarily varying Au nanorods can be well described using a coupled mechanical oscillator model. The coupled plasmon energy can be easily, yet accurately predicted with a universal hyperbolic formula. The nanorod heterodimers also exhibit Fano interference, which ii can be adjusted by controlling the gap distance and monomer plasmon energies.;I have also investigated the coupling between a Au nanorod and a small Au nanosphere approaching it. The plasmonic responses of the heterodimer, including Fano resonance, are remarkably sensitive to the nanosphere position on the nanorod, the gap distance, and the nanocrystal dimensions. Interestingly, the nanosphere dipole is found to rotate around the nanorod dipole to achieve favorable attractive interaction for the bonding dipole-dipole mode. The sensitive spectral response of the heterodimer to the spatial perturbation of the nanosphere offers an approach to designing plasmon rulers of two spatial coordinates for sensing and high-resolution measurements of distance changes.;Furthermore, the plasmon coupling can be modulated by an ultra-thin monolayer graphene. Coupled plasmonic structures can compress light energy into an ultra-small region, permitting a strong light-graphene interaction. Effective light modulation therefore can be realized by the graphene-metal hybrid plasmonic structures. I have explored the interaction between graphene and two different coupled plasmonic structures, including Au nanorod dimers and the Au film-coupled Au nanosphere structures. For nanorod dimers, no apparent spectral modulation can be acquired through graphene. By contrast, remarkable modulation on Au nanosphere scattering can be achieved by adding graphene into the cavity between the nanosphere and the supporting film. The graphene-loaded nanosphere antennas exhibit significant resonance redshifts, which can further be modulated when graphene screening effect is varied. This graphene-decorated plasmonic nanocavity not only pushes the optical response of graphene into the visible-to-near-infrared (NIR) region but also naturally exemplifies an electro-plasmonic system. These findings thus open an avenue on effectively operating graphene photonic devices in the visible-to-NIR range and pave a way for electrically controlling light by plasmonic structures. (Abstract shortened by UMI.).