Near-field Optical Microscopy


Ruiter, Anthonius Gerardus Theodorus (1997) Near-field Optical Microscopy. thesis.

Abstract:Near-field scanning optical microscopy (NSOM) is one of the most recent scanning probe techniques. In this technique, an optical probe is brought in the vicinity of the sample surface, in the near-field zone. The microscope can either work in illumination mode, in which the probe consists of a sub-wavelength light source, or in collection mode, in which the probe acts as a sub-wavelength detector. By scanning the probe over the sample surface and measuring the optical signal at each position, an optical image can be created. Because the probe has to be kept in the near-field zone at constant distance to the sample, in order to avoid intensity changes, a probe-sample distance regulation scheme is used to maintain a constant distance between probe and sample. The application of a distance regulation scheme results in the capability to measure a topographical image simultaneously with the optical image, an important asset of a near-field scanning optical microscope.
This thesis reports the development of two types of illumination mode near-field scanning optical microscopes. The microscopes use aperture type near-field probes, consisting of a sub-wavelength aperture which is illuminated from one side. The light transmitted through the aperture serves as sub-wavelength light source illuminating the sample.
Chapter 1 presents a brief introduction into the theory of optical image formation, leading to the resolution limit in conventional far-field optical microscopy. A near-field scanning optical microscope overcomes this limit by probing the near-field over the sample surface. A short overview of the instrumental and experimental accomplishments in nearfield optical microscopy is given in section 1.4. One of the most important parts of the near-field scanning optical microscope is the optical probe. In the experiments metal coated tapered fibers as well as newly designed cantilever probes, with a subwavelength aperture, have been used. Chapter 2 describes the fabrication process and emission characteristics of metal coated tapered optical fibers. Additionally, the micromechanical fabrication of a new type of probe, based on atomic force microscope probes, is described. The probe consists of a silicon nitride cantilever with a solid transparent conical tip. The probes are tested in a newly built near-field scanning optical microscope system. Although the prospects of using cantilever type probes are good, fiber probes are still favored because of the superior optical properties of the aperture.
The distance regulation scheme in a fiber based near-field microscope, the shear-force control mechanism, is examined in chapter 3. The dynamics of this shear-force feedback system, based on piezoelectric quartz tuning forks, has been investigated. In this system the fiber is attached to the tuning fork and excited externally at its resonance frequency. Experiments reveal that the resonance frequency of the tuning fork changes upon approaching the sample. Both amplitude and phase of the oscillation of the tuning fork can be used as distance control parameter in the feedback system. Using amplitude a second-order behavior is observed while with phase only a first-order behavior is
observed. The topography of a sample consisting of DNA strands on mica was imaged using phase feedback. A near-field scanning optical microscope with two polarization detection channels, operating with tuning fork shear-force feedback, has been used to observe rotational and translational diffusion of single molecules. The molecules were dispersed on glass or embedded in
polymer. In successive images the fluorescence of single molecules was followed over about one hour, with 10 ms integration time, until photodissociation. The orientation of the in-plane emission dipole of all molecules in one image could be directly determined with an accuracy of a few degrees. Different sets of molecules could be selectively excited by rotating the excitation polarization. Monitoring DiI molecules in PMMA over one hour, rotation of less than 10 degrees for the majority of molecules was found, while incidental fast rotation and transition to a dark state occurred. The fluorescence intensity was observed to be molecule dependent, which is an indication for out-of-plane orientation and different local photophysical environment.
Interactions between sample and probe, other than due to the light source character of the probe, have been observed in some of the single molecule experiments. Chapter 5 shows some examples of these interactions, such as sample manipulation by the probe and fluorescence quenching.
Finally, the results in this thesis are discussed in a broader perspective and an outlook into future developments in near-field optical microscopy is given.
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