Plasmonic properties of metallic nanostructures with reduced symmetry

Abstract

In this thesis, we theoretically study the plasmonic properties of metallic nanostructures with reduced symmetry using the Plasmon Hybridization (PH) and the Finite Difference Time Domain (FDTD) methods. Both methods provide efficient and accurate results for calculating physical properties of metallic nanostructures, including the optical cross section spectra, the local electromagnetic fields and induced charge densities around the surface of the nanostructures. The PH method is applied to a nanoshell with an offset core (nanoegg). The results show that the reduction in symmetry relaxes the selection rules in the hybridization of primitive plasmon modes, allowing for an admixture of dipolar components in higher multipolar plasmon modes of the particle. The hybridization therefore makes higher multipolar nanoshell plasmon modes dipole active, resulting in a core offset-dependent shift for the plasmon energies and a multipeaked feature in the optical spectrum. The polarization dependence of the optical absorption spectra is found to be relatively weak. The calculations also show significantly larger local-field enhancements on nanoegg's external surface than the equivalent concentric spherical nanostructure. The results agree very well with results from FDTD simulations and experiments, suggesting applications of nanoeggs as substrates for surface enhanced Raman spectroscopy (SERS) Another comprehensive investigation of the plasmonic interactions of individual metallic nanoshells with dielectric substrates is performed using the FDTD method. The results show that the adjacent dielectric breaks the spherical symmetry of individual nanoshell and lifts the degeneracy of the dipole and quadrupole plasmon modes, introducing significant polarization dependent redshifts and hybridization of the nanoparticle plasmon resonances. The results also show that, for small nanoparticle-substrate separations and substrates with large dielectric permittivities, the hybridized quadrupolar nanoparticle plasmon resonances also appear in the scattering spectrum. We discuss different numerical approaches in FDTD simulations for calculating the scattering spectrum in typical dark-field scattering geometries. We also discuss issues of numerical convergence and show that the scattering spectra can be calculated using finite substrate slab models. The results agree very well with experiments, showing that dielectric substrates matter in optical measurements of plasmonic nanoparticles. FDTD method is also applied to a bowtie-shaped nanostructure (nanobowtie). The calculations show significantly large SERS enhancements across a broad bandwidth of exciting wavelengths because of the complicated mode structure possible in the interelectrode gap. Nanometer-scale asperities in the gap area break the inter-electrode symmetry of the structure, resulting in optical excitations of many inter-electrode modes besides the simple dipolar plasmon mode commonly considered. The broken symmetry also leads to much less dependence of the calculated enhancement on polarization direction, as seen experimentally. The calculations confirm that the electromagnetic enhancement is confined in the normal direction to the film thickness and to a region comparable to the radius of curvature of the asperity. The calculated electromagnetic enhancements can exceed 1011, approaching that sufficient for single-molecule sinsitivity. We also compare the calculated extinction spectra for various values of interelectrode conductance connecting the source and drain. The results show that negligible charge transfer occurs between the two electrodes until junction conductance approaches the conductance quantum, G 0 = 2e2/h.