Plasmon Evolution in Core–Shell Nanospheroids

In order to explore the fundamental features of plasmon evolutions in the plasmonic nanostructures along variable geometrical parameters, TiO2–Ag core–shell nanospheroids, which have the distinguishable antibonding and bonding modes, are first used to illustrate the phenomena of plasmon evolutions by simulations. The usual peak-shift behaviors and appearance of the new modes are observed in the far-field extinction spectra. Beneath those phenomena, the unusual mode transformations occur in some modes. In the variable core configuration, when the inner surface of the silver shell is in a close proximity to the outer surface, the dipole antibonding mode evolves to be mixed with the quadrupole mode on the outer surface, while the new emerging mode evolves to the octupole antibonding modes. In the variable shell configuration, when the outer surface approaches the inner surface, the dipole bonding mode tends to evolve to the octupole bonding mode and the new modes emerge and tend to be the triakontadipole-like and octupole-like mode on the outer and inner surfaces. When the polar radius is so large that the outer surface is far away from the inner surface, the dipole antibonding mode evolves to the octupole antibonding mode and the new mode emerges which belongs to the octupole bonding mode. In mode transformation phenomena, one feature is that the evolution is associated with the odd l number (l = 1 for dipole, l = 3 for octupole, and l = 5 for triakontadipole) except for the mixed modes. Another feature is that the antibonding modes can evolve from the octupole to dipole and then octupole modes, in which process the charge distributions for the octupole modes are totally inverse. The retardation effects and the dielectric core effects are also discussed based on the phenomena of the higher order modes. The peak-shift behaviors, the appearance of the new modes, and the mode transformation along variable geometrical parameters have great importance in plasmonic applications due to the tunable resonance wavelength and the local field control.