Radiative effects in plasmonic aluminum and silver nanospheres and nanorods

We explore localized surface plasmon resonances in small (5-30 nm radius) aluminum and silver nanoparticles using classical electrodynamics simulations, focusing on radiative (farfield scattering) effects and the unique characteristics of aluminum as a plasmonic material. In Al spheres, higher-order plasmon resonances (e.g. quadrupoles) are significant at smaller sizes (> 15 nm) than in Ag spheres. Additionally, although the plasmon width is minimized at a radius of about 15 nm for both materials, the Al plasmon linewidth (similar to 1.4 eV) for the dipole mode is much larger than that observed in Ag (similar to 0.3 eV). The radiative contribution to damping dominates over non-radiative effects for small (5-20 nm) Al spheres (> 95%) whereas for similar size Ag spheres damping is almost entirely attributed to the bulk dielectric function (non-radiative). For Al nanorods the linewidths can be narrowed by increasing aspect ratio such that for an aspect ratio of 4.5, the overall Al (0.75 eV) linewidth is reasonably close to that of the same size Ag rod (0.35 eV). This narrowing arises from frequency dispersion in the real part of the Al dielectric function, and is associated with a 65% (1.5 to 0.5 eV) decrease in the radiative contribution to the linewidth for Al. Concurrently, an increase in the non-radiative width occurs as the aspect ratio increases and the plasmon tunes to the red. This demonstrates that anisotropy can be used as a parameter for controlling Al plasmon dephasing where the composition of the plasmon linewidth (radiative or non-radiative) can be tailored with aspect ratio. Overall, these data suggest that localized surface plasmon resonance dephasing mechanisms in Al nanostructures are inherently different from those in the noble metals, which could allow for new applications of plasmonic materials, tunable plasmon lifetimes, and new physics to be observed.