Plasmon resonances of silver nanowires with a nonregular cross section

TLDR

We investigate numerically the plasmon resonance spectrum of metallic nanowires with nonregular cross sections in the 20–50 nm range. We numerically compute resonance spectra for nanowires with various simplex cross sections and evaluate the average Raman enhancement for molecules uniformly covering the particle surface. Reducing cross‑section symmetry increases the number of resonances from one in cylindrical wires to more than five in triangular ones, broadening the spectral range and producing distinct colors; at resonance, field enhancements up to several hundred times the incident field yield SERS boosts exceeding 10¹², with the distance dependence governed by the specific plasmon resonance.

Abstract

We investigate numerically the spectrum of plasmon resonances for metallic nanowires with a nonregular cross section, in the $20--50 \mathrm{nm}$ 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 (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.