Constant-height scanning is demonstrated to improve near-field microsc
opy by eliminating artifacts connected with topography scanning, hence
, to image the inherent electromagnetic contrast. Microwaves are chose
n for this study because the long wavelength eliminates coherence arti
facts, owing to a scale separation of wave and image frequencies. Meas
ured amplitude and phase images of conductive films are quantitatively
analyzed by considering the longitudinal electric near field. The obs
erved spatial resolution of 200 nm equals the probing tip size and pro
ves that the skin depth delta of the tip material (here, 1600 nm) pres
ents no resolution limit to scanning optical microscopy. (C) 1997 Amer
ican Institute of Physics.