Manipulating surface plasmon of metallic nanoparticles and applications on near-field optical disk and fiber-optic biosensor

[[abstract]]High local-field enhancement appears in the vicinity of the surface of metallic nanostructures due to surface plasmon excitation and therefore electromagnetic fields can be confined and controlled in nanoscale region. The controllable and tunable surface plasmon of metallic nanoparticles is studied analytically and numerically using Mie scattering theory and finite-difference time-domain method, respectively. For a single coated metallic nanoparticle, the surface plasmon excitation can be controlled by changing permittivity of the coated material, and furthermore the higher-mode enhancement of the coated metallic nanoparticle is observed. The surface plasmon due to near-field coupling of metallic nanoparticles is studied numerically by finite-difference time-domain method. For a silver nanocylinder pair with different radii of silver nanocylinders, asymmetric polarization charges are induced by the mutual interaction between nanocylinders. asymmetric confined charges are induced around the nanoscale gap by the mutual interaction between the nanocylinders. The field intensity in the gap associated with the density of confined charges around the gap and it can be controlled by interparticle distance, radius ratio of asymmetric pairs, and the illumination direction of incident light.Besides, the controllable and predictable surface plasmon resonance is demonstrated in three-pair array structures. A simplified open cavity model is proposed to understand the cavity-like resonant behavior of pair array structures. Recently, local-field enhancement of metallic nanoparticles has been applied to enhance the resolution and the sensitivity of near-field optical disks and fiber-optic biosensors, respectively. The random distributed silver nanoparticles which embedded in AgOx layer of an AgOx-type near-field optical disk become high scattering centers and can transfer the diffracted evanescent components from subwavelength recording marks into the propagating components that can be detected in far-field region. The numerical result shows that the recording marks smaller than wavelength/10 are distinguishable. A Fourier optics approach is used as a theoretical background to understand the subwavelength resolution capability of near-field optical disks. The influences of the distribution and density of metallic nanoparticles, and the illumination frequency of incident light on the resolution capability of near-field optical disks are also discussed. Finally, the mechanism of fluorescence signal enhancement of a localized surface plasmon coupled fluorescence fiber-optic biosensor with gold nanoparticles is studied by the scattering of evanescent waves by a single gold nanoparticle. Local-field enhancement appears in the vicinity of a gold nanoparticle when the nanoparticle is illuminated by evanescent waves from the surface of the uncladded fiber and the fluorescence signals of fluorophores which is bounded on the surface of the nanoparticle can be enhanced by the enhanced localfield. Calculated result shows that the averaged-field intensity around the gold nanoparticle is enhanced few times of the field intensity without nanoparticle and the field enhancement of the theoretical calculation is consistent with the experimental result.