Artifacts in scanning near-field optical microscopy (SNOM) due to deficient tips

Abstract

Apertureless scanning near-field optical microscopy (SNOM) in the reflection-back-to-the-fiber configuration is possible on rough surfaces of practical importance and reaches 15 nm lateral resolution. The feedback for constant distance mode is provided by shear-force atomic force microscopy. Any artifacts in the atomic force microscopy will translate into artificial optical responses. Very sharp tapered tips (radius ca. 15 nm, apex angle <10°) are required. Only these give the strongly enhanced reflectance in shear-force distance, do not break as easily as do more blunt tips, and can follow the topography in order to provide chemical SNOM contrast without topographic errors. In the absence of significant reflectance enhancement, if blunt or abrased or broken tips are used, not SNOM but artificial ‘optical contrast’ is recorded. Metal-coated tips are unsuitable: they become hot, give poor resolution and are the source of numerous additional artifacts that are summarized for comparison. The SNOM artifacts are subdivided into: topographic errors—if chemical uniform topography does give some optical response; lack of spatial correlation—if the optical signal does not reproduce the site of the emitter or of the chemical contrast; tip imaging—very large features and more or less negative differentials therefrom; contrast inversion—false sign of optical contrast; split optical contrast—signal is split into two unequal parts with the same (false) sign of the contrast; artificial stripes—instead of chemical/emissive contrast artificial stripes, that are not always ‘interference fringes’ or ‘edge-contrast’. The artifacts can be easily recognized in published images from different techniques and modes of SNOM. Their analysis would be facilitated if full x/y/z data were published instead of two-dimensional images. We therefore disclose interactively analyzable VRML data in the EPOC version. Copyright © 1999 John Wiley & Sons, Ltd.