Collective modes of void-surface coupled system

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Abstract
Mixing between two kinds of plasma oscillations, the void plasmons on a spherical void surface and the surface plasmons on a flat metal surface, is analyzed. Taking into account the dispersion relation of the surface plasmons, we find that bound states of collective modes appear when the void-surface distance is smaller than some threshold value. As the void approaches the surface, the binding energy and the localization of the induced charge are greatly enhanced due to the induced multipoles on the void surface. The resulting van der Waals attraction between the void and the surface is found to be larger (at least for a void in near contact with the surface) than the elastic interaction energy estimated from an existing theory of elasticity. To demonstrate the spectral density broadening due to the presence of the localized modes, the dipolar spectral density is calculated for various parameters.