Plasmon resonances of silver and gold nanorods

We studied the extinction and integral scattering spectra of gold and silver nanorods with random or ordered orientations. The calculations were carried out for spheroids, right circular cylinders, circular cylinders with hemispherical ends (T-matrix method), and the rectangular prisms (Discrete Dipole Approximation, DDA). For particles with equivolume diameters greater than 20 run, we have found a new resonance that is located between the usual plasmon resonances corresponding to the excitation of nanorods by the longitudinal or transversal electric fields. The new resonance can be excited only by TM incident wave, and its magnitude is maximal for orientation of the particle symmetry axis at 54degrees with respect to the light propagation direction. By contrast to the dominant scattering nature of the longitudinal resonance, the new resonance corresponds to the strong absorption comparable with scattering contribution to the total extinction. It is shown that the new resonance can be attributed to the quadrupole excitation of metal nanorods.