Dielectric versus topographic contrast in near-field microscopy

Using a fully vectorial three-dimensional numerical approach (generalized field propagator, based on Green's tensor technique), we investigate the near-field images produced by subwavelength objects buried in a dielectric surface. We study the influence of the object index, size, and depth on the near field. We emphasize the similarity between the near field spawned by an object buried in the surface (dielectric contrast) and that spawned by a protrusion on the surface (topographic contrast). We show that a buried object with a negative dielectric contrast (i.e., with a smaller index than its surrounding medium) produces a near-field image that is reversed from that of an object with a positive contrast. (C) 1996 Optical Society of America.