Abstract
In scanning near-field optical microscopy, the most popular probes are made of sharpened glass fiber attached to a quartz tuning fork (TF) and exploiting the shear force-based feedback. The use of tapping mode feedback could be preferable. Such an approach can be realized, for example, using bent fiber probes. Detailed analysis of fiber vibration modes shows that realization of truly tapping mode of the probe dithering requires an extreme caution. In case of using the second resonance mode, probes vibrate mostly in shear force mode unless the bending radius is rather small (ca. 0.3 mm) and the probe's tip is short. Otherwise, the shear force character of the dithering persists. Probes having these characteristics were prepared by irradiation of a tapered etched glass fiber with a CW CO2 laser. These probes were attached to the TF in double resonance conditions which enables achieving significant quality 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 of nonresonant excitation.
Highlights
Scanning near-field optical microscopy (SNOM) technique enables overcoming Abbe diffraction limit of far-field optics and obtaining simultaneously optical and topographical images; see, for example, [1,2,3] for recent reviews
In scanning near-field optical microscopy, the most popular probes are made of sharpened glass fiber attached to a quartz tuning fork (TF) and exploiting the shear force-based feedback
The most popular method of the SNOM tip-sample distance control is the shear force-based feedback employing a fiber attached to the quartz tuning fork (TF) first introduced in 1995 [5]
Summary
Scanning near-field optical microscopy (SNOM) technique enables overcoming Abbe diffraction limit of far-field optics and obtaining simultaneously optical and topographical images; see, for example, [1,2,3] for recent reviews. There is no such a significant difference between shear force and tapping modes When the former is operative, the oscillation occurs at somewhat unfavorable conditions with the angle between the velocity of the movement of the fiber probe tip and normal to the sample surface approaching ninety degrees. We report the realization of a new approach to the problem: bent sharpened glass optical fibers with carefully controlled (and small) sizes of the bent part and the radius of the curvature of the bending were prepared and experimentally exploited as SNOM probes The design of these probes has been based on detailed theoretical and numerical studies of the relative tip-sample surface motion. There are substantial grounds to suppose that the most of the earlier reported tapping mode SNOMs were not the devices exploiting the tapping mode
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