Effect Of Symmetry Breaking On Surface Plasmons In Metallic Nanoshell Arrays

Recently, symmetry breaking effect in Plasmonics has aroused intense interests. Many unique optical properties arise from symmetry-broken metallic nanostructures. In this thesis, we theoretically investigate the effect of symmetry breaking on localized and delocalized surface plasmon (SP) modes in monolayer hexagonal-close-packed metallic nanoshell arrays. The main conclusions of this work are as follows:1. When nanoshells are truncated into nanocups, we study how the localized spherelike and void SP modes evolve. The electric field of spherelike modes is mainly distributed outside the shells and forms several hot spots on the shell surface, the number of which is determined by the order of the spherelike mode. As the opening angle of nanoshells gradually increases, its resonance intensity and wavelength almost keep unchanged. However, when nanoshells are further truncated, the intensity decreases rapidly and the resonance eventually disappears. The void mode has its electric field highly confined within the metallic shell. As nanoshells are gradually truncated, the void mode suffers an increasing radiation loss. Consequently, its resonance intensity will also progressively decrease.2. Under higher degree of symmetry breaking, the coupling between the spherelike and void mode forms a hybridized SP mode. Although under this circumstance, the spherelike and void modes are only weakly excited, the strong coupling between them leads to an enhancement of SP resonance, which could be attributed to the large optical cross section of the void Mie mode. By tuning the dielectric constant of the media inside and outside nanocups, their coupling strength can be readily controlled. Also, the hybridization between different ordered spherelike and void modes is observed. Meanwhile, the resonant wavelength of the hybridized SP mode shows a dependence on the opening angle of nanocups, which can be approximately described by an intuitive SP standing wave model.3. We also study the symmetry breaking effect on the delocalized Bragg-type SP mode. This mode has more electric hot spots distributed outside the shell and thus, is more sensitive to the opening of nanoshells. So it disappears earlier than spherelike modes. However, interestingly, when the opening of nanoshells further increases, this Bragg-type mode reappears which could be demonstrated by its dispersion properties. We believe that via the near-field coupling between hot spots around the rims created by the opening, Bragg-type SP mode can be re-established. This near-field interaction based mechanism can be proved by the fact that as we gradually enlarge the lattice period, this mode eventually disappears.