Scanning near-field optical microscopy of metallic features

A numerical method has been used to model images acquired from a scanning near-lield optical microscope (SNOM) operating in illumination mode. Results are presented for samples consisting of small metallic features (strips, slots, islands) deposited onto a silica substrate. As anticipated, the model microscope is capable of resolving features that are almost arbitrarily small (similar to ~15 nm), however, it is found that the appearance of the modelled images is dependent on the conditions under which the image is captured and can show such counter-intuitive effects as 'contrast inversion' in which small metallic features appear as bright regions on the collected image. The results are contrasted with those from earlier studies which focussed mainly on dielectric structures.