Random surface plasmon eigenmodes revealed

Surface Plasmon (SP) eigenmodes of single metallic nanoparticles have spatial and spectral features which depend on the shape of the nanoparticle. For simple shapes, these properties follow clear trends with the size, constitutive material and dielectric environment of the nanoparticle which are now well-known. However, the situation is far more complex in disordered, or random metal-dielectric systems. A simple example of disordered medium is a semicontinuous metal film, obtained naturally when depositing some metal onto a dielectric substrate. Very broadband strong absorption features have been reported in semicontinuous metal films around the percolation threshold. Simple theoretical models related them to light induced randomly dispersed very intense electric fields named Hot Spots (HS). These HS were predicted to show peculiar properties, namely, a strong confinement, a random position over the substrate, and a strongly broadband character.

By using spatially resolved Electron Energy Loss Spectroscopy, we have precisely investigated the factors triggering the appearance of HS. Precisely, we have shown that they are the result of random, disorder-driven SP eigenmodes arising from the fractal character of semicontinuous films around the percolation threshold. These eigenmodes have been fully characterized and shown to strongly depart the SP eigenmodes of single nanoparticles. The results have just appeared in Phys. Rev. B.