Fluctuations of the Local Density of States Probe Localized Surface Plasmons on Disordered Metal Films

We measure the statistical distribution of the local density of optical states (LDOS) on disordered semicontinuous metal films. We show that LDOS fluctuations exhibit a maximum in a regime where fractal clusters dominate the film surface. These large fluctuations are a signature of surface-plasmon localization on the nanometer scale.

Figure 1

(a) TEM image of a sample with gold filling fraction $f=67\%$. The white circle indicates the size of a fluorescent bead. (b) Scheme of the experimental configuration. Excitation and detection are performed with a confocal microscope on a single fluorescent bead. A 40 nm thick silica spacer is deposited on each sample. (c) Typical decay curves recorded on two different beads on the sample in panel (a).

Figure 2

Decay rate distributions for two different samples. (a) $f=30\%$ (isolated nanoparticles). (b) $f=82\%$ (fractal clusters). Insets: TEM image of each sample.

Figure 3

(a) Normalized variance ${\sigma }^2(\Gamma )/⟨\Gamma {⟩}^2$ versus the gold filling fraction $f$. A sharp peak is visible for $f=82\%$ and is the signature of surface-plasmon modes localization. (b) Fractal parameter $S_f/S_e$ versus $f$. The dotted line is a guide for the eye. Inset: Perimeter versus surface of the clusters of a given sample ($f=79\%$) deduced from a TEM image (double logarithmic scale). The solid lines give the fractal dimension $D$ showing the existence of Euclidian ($D=1$) and fractal clusters ($D=1.88$).