Localized Plasmons in Noble Metal Nanospheroids

For the entire range of spheroidal noble metal nanoparticles, including spheres, rods, and disks, the optical properties are investigated as a function of their geometry and size by means of extinction spectra obtained by numerical methods. For spherical silver and gold particles, Mie theory is used to identify multipole plasmon resonances up to hexadecupole order as a function of particle radius. With increasing particle dimensions, higher-order multipole modes become more important. Moreover, the similarity in dielectric functions of silver and gold gives rise to almost identical optical properties at wavelengths exceeding 700 nm. For oblate and prolate nanoparticles, the dipole, quadrupole, and octupole plasmon resonances for aspect ratios up to 8 are considered. As with the nanospheres, extinction spectra for small nanodisks and nanorods are in agreement with the quasi-static approximation, whereas for larger dimensions, a red shift is observed. The plasmon peak positions are analyzed in terms of a material-independent factor that only depends on the geometry. We attempt to obtain a generalized description of the plasmon energies as a function of particle geometry and size. Finally, the size-dependent red shift is compared to surface plasmon dispersion relations and discussed in terms of surface curvature of the nanoparticle edge.