Evanescent wave scattering and local electric field enhancement at ellipsoidal silver particles in the vicinity of a glass surface.

We report on the numerical analysis of the local electric field enhancement of nanosized silver ellipsoids placed in the evanescent field near a glass surface. Across the visible spectrum the enhancement factor for silver particles varies by more than one order of magnitude because of surface-plasmon resonance. Because of the spatially inhomogeneous excitation, higher-order modes additionally contribute and modify the spectral dependence of the electric field compared with plane-wave excitation. Moving the metal particle toward the glass surface increases the field enhancement and shifts the plasmon resonance, which in addition splits between both ends of the particle. Besides the near-field properties of such a probe we also discuss to what extent these local properties can be measured in the far field.