Surface Plasmon States in Inhomogeneous Media at Critical and Subcritical Metal Concentrations

The excitation of collective electronic states, surface plasmons (SPs), is studied for semi-continuous metal films at various metal concentrations. A peculiarity in the optical response, manifested through a decrease in the exponents of the local field moments, is predicted at critical metal concentrations. This phenomenon directly results from an increased role of delocalized SP states at percolation. Experimental results from near-field optical microscopy are found to be in good agreement with calculations based on the theory, validating the role of SP localization in the optical response. Possible applications in improving the sensitivity of spectroscopic measurements such as surface enhanced Raman scattering and harmonic generation are considered.