IMAGE-CONTRAST OF DIELECTRIC SPECIMENS IN TRANSMISSION MODE NEAR-FIELD SCANNING OPTICAL MICROSCOPY - IMAGING PROPERTIES AND TIP ARTIFACTS

Near-field scanning optical microscopy (NSOM) is a scanned probe techn ique utilizing a subwavelength-sized light source for high-resolution imaging of surfaces, Although NSOM has the potential to exploit and ex tend the experimental utility of the modern light microscope, the inte rpretation of image contrast is not straightforward. In near-field mic roscopy the illumination intensity of the source (probe) is not a cons tant value, rather it is a function of the probe-sample electronic env ironment. A number of dielectric specimens have been studied by NSOM t o elucidate the contrast role of specimen type, topography and crystal linity; a summary of metallic specimen observations is presented for c omparative purposes. Near-field image contrast is found to be a result of lateral changes in optical density and edge scattering for specime ns with little sample topography, For surfaces with considerable topog raphy the contributions of topographic (Z) axis contrast to lateral (X ,Y) changes in optical density have been characterized. Selected near- field probes have also been shown to exhibit a variety of unusual cont rast artefacts. Thorough study of polarization contrast, optical edge (scattering) contrast, as well as molecular orientation in crystalline specimens, can be used to distinguish lateral contrast from topograph ic components. In a few cases Fourier filtering can be successfully ap plied to separate the topographic and lateral contrast components.