Spectral manifestation of collective plasma oscillations quasi-resonant to eigenfrequencies of individual nanoparticles in a silver Island film

A doublet is revealed in the light extinction spectrum of a thin silver island film and the explanation of its nature is proposed. Based on the electron microscopy data, it is found that the doublet cannot be formed only by intrinsic resonances of individual islands. Because of interisland interactions, light can excite collective fields localized near the film. Resonant frequencies of these localized modes depend on the size, concentration, and arrangement of individual islands, whose plasma oscillation frequencies differ only slightly from one another.