We investigate numerically the spectrum of plasmon resonances for metallic nanowires with a nonregular cross section, in the 20-50 nm range. We first consider the resonance spectra corresponding to nanowires whose cross sections form different simplexes. The number of resonances strongly increases when the section symmetry decreases: A cylindrical wire exhibits one resonance, whereas we observe more than five distinct resonances for a triangular particle. The spectral range covered by these different resonances becomes very large, giving to the particle-specific distinct colors. At the resonance, dramatic field enhancement is observed at the vicinity of nonregular particles, where the field amplitude can reach several hundred times that of the illumination field. This near-field enhancement corresponds to surface-enhanced Raman scattering ͑SERS͒ enhancement locally in excess of 10 12 . The distance dependence of this enhancement is investigated and we show that it depends on the plasmon resonance excited in the particle, i.e., on the illumination wavelength. The average Raman enhancement for molecules distributed on the entire particle surface is also computed and discussed in the context of experiments in which large numbers of molecules are used.

We study the coupling induced by retardation effects when two plasmon-resonant nanoparticles are interacting. This coupling leads to an additional resonance, the strength of which depends on a subtle balance between particle separation and size. The scattering cross section and the near field associated with this coupled resonance are studied for cylindrical particles in air and in water. Implications for surface-enhanced Raman scattering and nano-optics are discussed. © 2001 Optical Society of America OCIS codes: 240.6680, 240.5420, 290.5860, 230.1150, 260.3910, 350.3950. It is a well-known phenomenon that small individual metallic particles of specific metals, such as gold and silver, can support plasmon resonances in the optical wavelength range.

We investigate numerically the plasmon resonances of 10±50 nm nanowires with a non-elliptical section. Such wires have a much more complex behavior than elliptical wires and their resonances span a larger frequency range. The ®eld distribution at the surface of these wires exhibits a dramatic enhancement, up to several hundred times the incident ®eld amplitude. These strongly localized ®elds can provide an important mechanism for surface enhanced Raman scattering (SERS). Ó

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