Coexistence of Localized and Delocalized Surface Plasmon Modes in Percolating Metal Films

Near-field intensity statistics in semicontinuous silver films over a wide range of surface coverage are investigated using near-field scanning optical microscopy. The variance of intensity fluctuations and the high-order moments of intensity enhancement exhibit local minima at the percolation threshold. This reduction in local field fluctuations results from resonant excitation of delocalized surface plasmon modes. By probing the modification of the critical indices for high-order moments of intensity enhancement caused by the delocalized states, we provide the first experimental evidence for the coexistence of localized and delocalized surface plasmon modes in percolating metal films.

Figure 1

Near-field intensity variance $\mathrm{var}(I)$ as a function of silver concentration $\Delta p=p-p_c$, obtained experimentally (solid line with triangles) and numerically with (dotted line) and without (dashed line) spatial averaging.

Figure 2

High-order moments of near-field intensity enhancement $M_{2n}$ as a function of silver concentration $\Delta p=p-p_c$, obtained experimentally (a) and numerically with (b) and without (c) spatial averaging.

Figure 3

(a) Log-linear plot of measured high-order moments of near-field intensity enhancement $M_{2n}$ vs $n$. (b) Slope of $\log (M_{2n})$ over $n$ obtained numerically after spatial averaging versus averaging parameter $d$. The measured slopes are marked by horizontal lines with the standard error given in the legend.