Plasmon hybridization in real metals

Abstract

By treating free electrons in metallic nanostructures as incompressible and irrotational fluid, Plasmon hybridization (PH) method can be used as a very useful tool in interpolating the electric magnetic behaviors of complex metallic nanostructures. Using PH theory and Finite Element Method (FENI), we theoretically investigated the optical properties of some complex nanostructrus including coupled nanoparticle aggregates and nanowires. We investigated the plasmonic properties of a symmetric silver sphere heptamer and showed that the extinction spectrum exhibited a narrow Fano resonance. Using the plasmon hybridization approach and group theory we showed that this Fano resonance is caused by the interference of two bonding dipolar subradiant and superradiant plasmon modes of E1u symmetry. We investigate the effect of structural symmetry breaking and show that the energy and shape of the Fano resonance can be tuned over a broad wavelength range. We show that the wavelength of the Fano resonance depends very sensitively on the dielectric permittivity of the surrounding media. Besides heptamer, we also used plasmon hybridization method and finite element method to investigate the plasmonic properties of silver or gold nano spherical clusters. For symmetric clusters, we show how group theory can be used to identify the microscopic nature of the plasmon resonances. For larger clusters, we show that narrow Fano resonances are frequently present in their optical spectra. As an example of asymmetric clusters, we demonstrate that clusters of four identical spherical particles support strong Fano-like interference. This feature is highly sensitive to the polarization of the incident electric field due to orientation-dependent coupling between particles in the cluster. Nanowire plasmons can be launched by illumination at one terminus of the nanowire and emission can be detected at the other end of the wire. With PH theory we can predict how the polarization of the emitted light depends on the polarization of the incident light. Depending on termination shape, a nanowire can serve as either a polarization-maintaining waveguide, or as a polarization-rotating, nanoscale half-wave plate. We also investigated how the properties of a nearby substrate modify the excitation and propagation of plasmons in subwavelength silver wires.