Influence of secondary tip shape on illumination-mode near-field scanning optical microscopy images

We report illumination-mode near-field optical microscopy images of individ ual 80-115-nm-diameter Au particles recorded with metal-coated fiber probes . It is found that the images are strongly influenced by the metal-coating thickness. This dependence is consistent with theoretical models, which are in good agreement with the experimental images. Probes with thick coatings (similar to lambda/2) produce images consisting of three extrema, owing to a resonancelike polarization of the probe end. Probes with thinner coating s generally produce simpler images. However, in some cases the images conta in wavelike structures that are due to interference between direct radiatio n from the tip and propagating tip fields scattered by the particles. (C) 1 999 Optical society of America [S0740-3232(99)01208-9].