Plasmon resonances in nanowires with a non-regular cross-section

We investigate numerically the spectrum of plasmon resonances for metallic nanowires with a non-regular cross-section in the 20-50 nm range. After briefly recalling the physical properties of metals at optical frequencies, we point out the intrinsic difficulties in the computation of the plasmon resonances for nanoparticles with a non-regular shape. We then 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 5 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 non-regular particles, where the field amplitude earl reach several hundred times that of the illumination field. This near-field enhancement corresponds to surface enhanced Raman scattering (SERS) 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. Finally we discuss the influence of the model permittivity which enters the calculation, as well as the resonances shift and broadening produced by a water background