Plasmon hybridization in complex metallic nanostructures
Nordlander, Peter J.
Doctor of Philosophy
With Plasmon Hybridization (PH) and Finite-Difference Time-Domain (FDTD) method, we theoretically investigated the optical properties of some complex metallic nanostructures (coupled nanoparticle/wire, nanostars, nanorings and combined ring/disk nanocavity systems). We applied the analytical formulism of PH studying the plasmonic coupling of a spherical metallic nanoparticle and an infinite long cylindrical nanowire. The plasmon resonance of the coupled system is shown shifted in frequency, which highly depends on the polarization of incident light relative to the geometry of the structure. We also showed the nanoparticle serves as an efficient antenna coupling the electromagnetic radiation into the low-energy propagating wire plasmons. We performed an experimental and theoretical analysis of the optical properties of gold nanorings with different sizes and cross sections. For light polarized parallel to the ring, the optical spectrum sensitively depends on the incident angle. When light incidence is normal to the ring, two dipolar resonance is observed. As the incident light is titled, some previously dark mulipolar plasmon resonances will be excited as a consequence of the retardation. The concept of plasmon hybridization is combined with the power of brute-force numerical methods to understand the plasmonic properties of some very complicated nanostructures. We showed the plasmons of a gold nanostar are a result of hybridization of the plasmons of the core and the tips of the particle. The core serves as a nanoantenna, dramatically enhanced the optical spectrum and the field enhancement of the nanostar. We also applied this method analyzing the plasmonic modes of a nanocavity structure composed of a nanodisk with a surrounding minoring. For the concentric combination, we showed the nature of the plasmon modes can be understood as the plasmon hybrization of an individual ring and disk. The interation results in a blueshifted and broadened superradiant antibonding resonance and a redshifted and narrowed subradiant bonding plasmon. The electric field enhancement of the subradiant mode is significantly larger compared with its parent plasmon modes. For the nonconcentric ring/disk nanocavity, we showed the symmetry breaking caused the coupling betweem different multipolar plamons which results in a tunable Fano resonance. We also show the subradiant and the Fano resonances could be particularly useful in the LSPR and SERS sensing applications. In the thesis, we also presented an efficient dielectric function of gold and silver that is suitable for the FDTD simulations of the optical properties of various nanoparticles. The new dielectric function is able to account for the interband transition in gold and silver, and provides more precise calculations of the optical spectra compared to the Drude dielectric function that is normally used previously.
Condensed matter physics; Optics; Theoretical physics