Simultaneous topographical and optical characterization of near-field optical aperture probes by way of imaging fluorescent nanospheres

We introduce a method for a simultaneous topographical and optical characterization of aperture probes for scanning near-field optical microscopy which is based on imaging of small sized fluorescent nanospheres (∼20 nm). The near-field optical fluorescence image of a nanosphere maps the intensity distribution of light at the end face of the probe whereas the simultaneously taken height image contains information about the aperture-sample distance. We used this method to control a mechanical modification of a near-field probe. By squeezing a probe repeatedly against a smooth glass substrate and thereby removing obstructing protrusions the aperture was brought as close as possible to the sample surface which resulted in a strongly improved optical resolution