Scattering of surface-plasmon polaritons by dipoles near a surface: Optical near-field localization

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Abstract
Electromagnetic radiation of electric dipoles, which are placed near a metallic surface and excited by the ongoing surface polaritons, is investigated. The dipoles are used to represent the surface scattering centers, irregulars, and mesoscopic particles. A formalism which distinguishes the evanescent field and propagating waves in the dipole radiation is employed to calculate the scattered surface polariton field in the near- and far-field zones. Numerical studies of the local field at the dipole sites were carried out for 50 dipoles with various distances between the dipoles and between the dipoles and the surface. An enhanced local field was obtained in some cases, and the conditions for the enhancement are discussed. Two-dimensional intensity distributions of the scattered field in the plane perpendicular to the surface and in the plane parallel to the surface for systems with up to 100 dipoles are presented for the propagating waves, the evanescent field, and the total field. Finally, a scanning local probe is introduced in the self-consistent calculations, and a numerical modeling of the near-field optical microscopy over the dipole system excited by the surface polaritons is carried out for various tip-surface separations. The results are employed to discuss phenomena in the surface polariton optics, particularly the recently observed strong optical near-field localization.