Control of the tip-surface distance in near-field optical microscopy

An experimental technique that makes use of the intensity of the interference pattern formed by light that propagates directly from the single-mode fiber tip and light that is reflected by the surface under anoblique angle of incidence is developed to control the tip-surface distance in near-field opticalmicroscopy. It is shown that by using another fiber as a detector with a polished edge placed at the surface near the fiber tip one can determine the tip-surface separation with an accuracy better than 15 nm at distances less than 1 µm. The technique proposed is used to investigate the influence of the shape of the tip in near-field measurements.