Dynamic force distance control suited to various probes for scanning near-field optical microscopy

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Abstract
A dynamic force distance control for scanning near-field optical microscopy (SNOM) based on a quartz tuning fork as piezoelectric force sensor is introduced. In contrast to a similar design for shear-force feedback, the tuning fork is aligned in such a way that forces perpendicular to the surface are detected. Various near-field probes can be attached to the end of the tuning fork and serve as force sensing tip. The high force sensitivity is demonstrated for two different near-field probes by imaging the topography of organic samples. The tetrahedral tip, an apertureless high-resolution near-field probe used up to now mainly with tunnel current distance control, is for the first time successfully combined with a force distance control for SNOM. A similar distance control was used in conjunction with an only few millimeters short aluminum-coated tapered fiber tip as near-field probe. The suitability of this design for near-field optical fluorescence imaging is demonstrated. Furthermore, the dynamic force distance control with a fiber tip was applied to a soft biological sample under water. The force sensitivity turned out to be sufficiently high to reveal corrugations in the order of 1 nm.