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Superfocusing of plasmon polariton on rough surface of metal: theory and experiment

It is well known that the roughness of the metal surface promotes the enhancement of the nonlinear optical response. It may be suggested that the increase of the wave field is caused by the excitement of the surface plasmon polaritons. However, up to now the peculiarities of the formation of polariton modes and causes of the anomalous growth of the wave fields have not been investigated. In the present work we consider the structure, which consists of two identical metallic cones, contacting by their tips. Theoretical considerations show that in such media the surface plasmon polariton has the following principal features: in approaching to the contacting point the wave vector of the polariton infinitely increases by the law of /spl sim/1/r, where r is distance to contact point and, according by its wavelength decreases. Then, the diffraction phenomena do not limit the localization of the wave in very small regions of the space. As a result, the wave fields anomalously grow. The combination of these two features presents the essence of the superfocusing of the surface polariton. In the experimental study we proceed from the supposition, that the mechanical contact between two pressed silver layers can be realized in separate points, that is the result of surface roughness, which have nanometric dimensions in the case of mirror surface. The simplest result of the superfocusing can be the photodamage of the surface in the neighborhood of contact points.