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The tetrahedral tip is used as a light emitting probe for scanning near-field optical microscopy (SNOM). It has no aperture as an element for the confinement of light and the techniques of scanning tunneling microscopy and SNOM can be combined with the same probing tip. Silver grains are distinguished from gold grains by their specific near-field optical contrast in SNOM transmission mode images of mixed films of silver and gold at a lateral resolution in the nanometer range and an edge resolution of 1 nm for selected grains. The contrast is explained in terms of a quasielectrostatic model of a local light-emitting source interacting with the object.
Koglin et al. (Sat,) studied this question.