True Tapping Mode Scanning Near-Field Optical Microscopy with Bent Glass Fiber Probes

In scanning near-field optical microscopy, the most popular probes are made of sharpened glass fiber attached to a quartz tuningfork (TF) and exploiting the shear force-based feedback. The use of tapping mode feedback could be preferable. Such an approachcan be realized, for example, using bent fiber probes. Detailed analysis of fiber vibration modes shows that realization of trulytapping mode of the probe dithering requires an extreme caution. In case of using the second resonance mode, probes vibratemostly in shear force mode unless the bending radius is rather small (ca. 0.3 mm) and the probe’s tip is short. Otherwise, the shearforce character of the dithering persists. Probes having these characteristics were prepared by irradiation of a tapered etched glassfiber with a CW CO2laser. These probes were attached to the TF in double resonance conditions which enables achieving significantquality factor (4000–6000) of the TF + probe system (Cherkun et al., 2006). We also show that, to achieve a truly tapping character,dithering, short, and not exceeding 3 mm lengths of a freestanding part of bent fiber probe beam should also be used in the case ofnonresonant excitation