Longitudinal plasmon modes of gold nanorod

The longitudinal plasmon modes of an elongated gold nanorod (GNR) excited by a pair of dipoles (bi-dipole) are studied, where these dipoles are located at the two ends of GNR respectively. The dipole moments are assumed to be aligned along the long axis of GNR. The electromagnetic fields induced by the bi-dipole in the presence of the GNR are calculated using the multiple multipole method. Two configurations of the anti-symmetric and symmetric bi-dipoles are discussed. The radiative and nonradiative powers of bi-dipole indicate these modes. Moreover, the far-field radiation patterns of these modes are discussed. For example, the first (dipole), second (quadrupole) and third (sextupole) modes of GNR (radius: 30 nm, aspect ratio: 8) in air are at 1520 nm, 800 nm and 610 nm, respectively. The scattering and absorption cross sections of GNR irradiated by an obliquely incident plane wave are also calculated for comparison. Our results show that it is much easier to induce the dipole, quadrupole and sextupole modes of GNR by the excitation of the bi-dipole than by that of the plane wave.