Direct interpretation of near-field optical images.

The interpretation of the detection process in near-field optical microscopy is reviewed on the basis of a discussion about the possibility of establishing direct comparisons between experimental images and the solutions of Maxwell equations or the electromagnetic local density of states. On the basis of simple physical arguments, it is expected that the solutions of Maxwell equations should agree with images obtained by collecting mode near-field microscopes, while the electromagnetic local density of states should be considered to provide a practical interpretation of illumination mode near-field microscopes. We review collecting mode near-field microscope images where the conditions to obtain good agreement with the solutions of Maxwell equations have indeed been identified. In this context of collecting mode near-field microscopes, a fundamentally different functionality between dielectric and gold-coated tips has been clearly identified experimentally by checking against the solutions of Maxwell equations. It turns out that dielectric tips detect a signal proportional to the optical electric field intensity, whereas gold-coated tips detect a signal proportional to the optical magnetic field intensity. The possible implications of this surprising phenomenon are discussed.