ALLOWED AND FORBIDDEN LIGHT IN NEAR-FIELD OPTICS .2. INTERACTING DIPOLAR PARTICLES

Near-field optical imaging mechanisms are investigated on an elementar y level with the use of a coupled dipole formalism. Both optical probe and sample particles are considered as single dipolar cells. The samp le particles are located on the surface of a layered substrate, and th e optical probe is guided over the sample at constant height. The theo ry of the coupled dipole formalism with the use of Green's functions o f a layered reference system is outlined, and asymptotic forms for far -field radiation are derived. Depending on the direction of observatio n, the recorded far-field radiation contains different information. It is shown that radiation emitted into the lower half-space at angles w ithin the critical angle of total internal reflection (allowed light) provides reliable images that are only weakly correlated to the topogr aphy of the sample. Higher resolution is achieved with the radiation e mitted at supercritical angles (forbidden light), but the high sensiti vity to topographical variations makes image interpretation difficult. In order to image the optical properties of the sample, it is shown t o be unfavorable to include the forbidden light in the optical signal. (C) 1997 Optical Society of America.