Field enhancement by a metallic sphere on dielectric substrates

We calculate the scattered local held around a small gold sphere on dielectric substrates which is illuminated by a collimated monochromatic beam. The numerical results show the field between the sphere and the substrate is much enhanced when the sphere is illuminated by a p-polarized beam. The enhanced area is very localized between the sphere and the substrate and the diameter of the enhanced area on the surface of the substrate is one fifth of the sphere diameter. The held increases as the gap distance decreases or as the refractive index of the substrate increases.