Near-field optical imaging mechanisms are investigated on an elementary level with the use of a coupled dipole formalism. Both optical probe and sample particles are considered as single dipolar cells. The sample particles are located on the surface of a layered substrate, and the optical probe is guided over the sample at constant height. The theory of the coupled dipole formalism with the use of Green's functions of a layered reference system is outlined, and asymptotic forms for far-field radiation are derived. Depending on the direction of observation, the recorded far-field radiation contains different information. It is shown that radiation emitted into the lower half-space at angles within the critical angle of total internal reflection (allowed light) provides reliable images that are only weakly correlated to the topography of the sample. Higher resolution is achieved with the radiation emitted at supercritical angles (forbidden light), but the high sensitivity to topographical variations makes image interpretation difficult. In order to image the optical properties of the sample, it is shown to be unfavorable to include the forbidden light in the optical signal.
© 1997 Optical Society of America
Lukas Novotny, "Allowed and forbidden light in near-field optics. II. Interacting dipolar particles," J. Opt. Soc. Am. A 14, 105-113 (1997)