Optical characterization of probes for photon scanning tunnelling microscopy

The photon scanning tunnelling microscope is a well-established member of the family of scanning near-field optical microscopes used for optical imaging at the subwavelength scale. The quality of the probes, typically pointed uncoated optical fibres, used is however, difficult to evaluate in a direct manner and has most often been inferred from the apparent quality of recorded optical images. Complicated near-field optical imaging characteristics, together with the possibility of topographically induced artefacts, however, has increased demands for a more reliable probe characterization technique. Here we present experimental results obtained for optical characterization of two different probes by imaging of a well-specified near-field intensity distribution at various spatial frequencies. In particular, we observe that a sharply pointed dielectric probe can be highly suitable for imaging when using p-polarized light for the illumination. We conclude that the proposed scheme can be used directly for probe characterization and, subsequently, for determination of an optical transfer function. which would allow one to deduce from an experimentally obtained image of a weakly scattering sample the field distribution existing near the sample surface in the absence of the probe.