Allowed and forbidden light in near-field optics I A single dipolar light source

The radiation of an arbitrarily oriented dipole located above a planarly layered structure is investigated. The dipole is regarded as a tiny light source, and the properties of the layer are varied in order to study the influence on far-field radiation. Topographic contrast is investigated by varying the thickness of the layer, phase contrast is investigated by varying its dielectric constant, and amplitude contrast is investigated by varying its absorption. It is shown that the light emitted into the lower half-space is composed of two major contributions. Radiation emitted into directions within the critical angle of total internal reflection (allowed light) behaves in a classical way, i.e., the contrast mechanisms are similar to those produced by far-field illumination. On the other hand, radiation emitted at supercritical angles (forbidden light) is exponentially dependent on the height of the dipole above the layer, and the contrast mechanisms turn out to depend sensitively on the spatial source spectrum (orientation of the dipole). Because of their different behavior, it is found to be unfavorable to detect both allowed and forbidden light in near-field optical microscopy.