Infoscience

Thesis

Conception and realization of a compact scanning near-field optical microscope

Combining the subwavelength resolution of near-field interactions with the optical contrast mechanisms of classical optical microscopy makes scanning near-field optical microscopy an interesting technique for many applications. In spite of more than ten years of development, the quality of this technique is still limited by technical inadequacies. Rethinking of the tip quality, the tip-sample distance control, the scanner and the general assembly may help to overcome these limitations. This thesis is dedicated to instrumental aspects of scanning near-field optical microscopy. This concerns mainly SNOM-tip characterization, tip-sample distance control by shear-force, and the implementation of a compact SNOM-head on an inverted optical microscope. Near-field probes based on etched fiber tips are produced with a modified etching technique, that leads to smoother tip surfaces. These probes are characterized with respect to their transmission, possible light leakage and tested on a calibrated object. We propose a new tip coating, based on monofunctional silylation, and tested for its properties under shear-force distance control. A simple and easy-to-use combined near- and far-field method is presented to evaluate aperture near-field probes concerning transmission ratio, position of light leakage in the metal coating, size of the aperture and geometric properties. Shear-force distance curves are discussed under several aspects such as curve shape, approach course and approach direction. The analysis of these data together with theoretical considerations concerning the forces acting on the SNOM-tip allow one to discuss the related importance of the different forces. In an aqueous environment viscous forces are shown to play an important role. A compact near field optical microscope module is presented, with shear force detection based on an external cavity laser interferometer and an easy-to-handle tip fixation system, involving standard commercially available elements. For investigations in anaqueous environment a new type of liquid cell is proposed, allowing shear-force distance control independent from the immersion depth. As a result from a comparison between different scanner configurations a new compact concept for an electromagnetic scanner with a flat parallel structure is presented. A transmission-mode SNOM instrument is realized by mounting the compact shear-force detection module and a feedback-controlled xyz -piezo scanner on a commercial inverted microscope. The performances of this system with respect to sensitivity, scanning range, long-term stability and the possibility to work in aqueous environments, allow comfortable and easy instrument handling, especially for investigations of biological samples.

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