Collective electronic states in inhomogeneous media at critical and subcritical metal concentrations

The excitation of collective electronic states, surface plasmons (SPs), is studied for semicontinuous metal films at various metal concentrations. A previously unexpected strong optical response, manifested through an increase in the exponents of the local field moments, is predicted at noncritical metal concentrations. This phenomenon results from an increase in SP localization away from the percolation threshold, which is opposite to the general understanding that a decrease in the number of scatters leads to weaker mode localization. Experimental results from near-field optical microscopy are found to be in good agreement with 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.