A reflection-mode apertureless scanning near-field optical microscope developed from a commercial scanning probe microscope
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We have developed a polyvalent reflection-mode apertureless scanning near-field optical microscope (SNOM) from a commercial scanning probe microscope (SPM). After having explained our motivations, we describe the instrument precisely, by specifying how we have integrated optical elements to the initial SPM, by taking advantage of its characteristics, and without modifying its initial functions. The instrument allows five different reflection-mode SNOM configurations and enables polarization studies. Three types of SNOM probes can be used: dielectric, semiconducting, and metallic probes. The latter are homemade probes whose successful use, as probes for atomic force microscopy, by the commercial SPM has been experimentally demonstrated. Using silicon–nitride (dielectric) probes, one of the five configurations has been experimentally tested with two samples. The first sample is made of nanometric aluminum dots on a glass substrate and the second sample is the output front facet of a laser diode. The preliminary SNOM images of the latter reveal pure optical contrasts.Keywords:
Scanning Probe Microscopy
Near-field optics
The resolution of various scanning probe microscopy methods can be applied to the fabrication of nanostructures. Various methods of local material modification based on different microscopic mechanisms have been proposed, examples of which are : material transfer between a scanning tunneling microscope (STM) tip and a substrate, local oxidation of silicon using atomic force microscope (AFM). Scanning near-field optical microscopy (SNOM) is also an attractive candidate for nanofabrication. Here the optical spot size in the near-field is given by the resolution of the SNOM which in turn is determined by the details of the tip geometry and is typically between 50 and 100 nanometers.
Scanning Probe Microscopy
Near-field optics
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The tetrahedral tip is introduced as a new type of a probe for scanning near‐field optical microscopy (SNOM). Probe fabrication, its integration into a scheme of an inverted photon scanning tunnelling microscope and imaging at 30 nm resolution are shown. A purely optical signal is used for feedback control of the distance of the scanning tip to the sample, thus avoiding a convolution of the SNOM image with other simultaneous imaging modes such as force microscopy. The advantages of this probe seem to be a very high efficiency and its potential for SNOM at high lateral resolution below 30 nm.
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We have developed a new easy-to-use probe that can be used to combine atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM). We show that, using this device, the evanescent field, obtained by total internal reflection conditions in a prism, can be visualized by approaching the surface with the scanning tip. Furthermore, we were able to obtain simultaneous AFM and SNOM images of a standard test grating in air and in liquid. The lateral resolution in AFM and SNOM mode was estimated to be 45 and 160 nm, respectively. This new probe overcomes a number of limitations that commercial probes have, while yielding the same resolution.
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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.
Near-field optics
Aperture (computer memory)
Scanning Probe Microscopy
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Abstract Scanning near-field optical microscope (SNOM) is hybridized with scanning tunneling microscope (STM) in order to achieve a higher spatial resolution by introducing a doubly metal-coated optical fiber tip with a nm-scale aperture. The result of a simultaneous SNOM/STM imaging of Au(111) indicates the boundary-sensitive detection in SNOM mode, which is not an artifact caused by z-motion crosstalk.
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Scanning near-field optical microscope (SNOM) is hybridized with a scanning tunneling microscope (STM) to investigate nanoscopic optical phenomena in both the near-field region and its proximity. The system is realized by introducing a doubly metal-coated optical fiber tip with an extremely small aperture (<100 nm), where the metal is coated on the aperture to obtain a half-transparent conducting tip after the fabrication of an “aperture probe.” A simultaneous SNOM/STM observation is performed for an Au (111) surface, where the evanescent field at the tip vicinity through the aperture is scattered by the local structures of the sample and the far-field component of the scattered light is collected as an optical signal. The distance control is carried out under the constant-current condition in order to separate the optical properties from surface topography. An optical resolution of λ/ 100 and identical channel transport for both electrons and photons are achieved. The intensity changes, as a function of the gap distance, are also measured in the far-field and the near-field regions and the proximity.
Near-field optics
Aperture (computer memory)
Scanning Probe Microscopy
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We have developed a polyvalent reflection-mode apertureless scanning near-field optical microscope (SNOM) from a commercial scanning probe microscope (SPM). After having explained our motivations, we describe the instrument precisely, by specifying how we have integrated optical elements to the initial SPM, by taking advantage of its characteristics, and without modifying its initial functions. The instrument allows five different reflection-mode SNOM configurations and enables polarization studies. Three types of SNOM probes can be used: dielectric, semiconducting, and metallic probes. The latter are homemade probes whose successful use, as probes for atomic force microscopy, by the commercial SPM has been experimentally demonstrated. Using silicon–nitride (dielectric) probes, one of the five configurations has been experimentally tested with two samples. The first sample is made of nanometric aluminum dots on a glass substrate and the second sample is the output front facet of a laser diode. The preliminary SNOM images of the latter reveal pure optical contrasts.
Scanning Probe Microscopy
Near-field optics
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The tetrahedral tip is used as a light emitting probe for scanning near-field optical microscopy (SNOM). It has no aperture as an element for the confinement of light and the techniques of scanning tunneling microscopy and SNOM can be combined with the same probing tip. Silver grains are distinguished from gold grains by their specific near-field optical contrast in SNOM transmission mode images of mixed films of silver and gold at a lateral resolution in the nanometer range and an edge resolution of 1 nm for selected grains. The contrast is explained in terms of a quasielectrostatic model of a local light-emitting source interacting with the object.
Near-field optics
Aperture (computer memory)
Scanning Probe Microscopy
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The near field of an apertureless near-field scanning optical microscopy probe is investigated with a multiple-multipole technique to obtain optical fields in the vicinity of a silicon probe tip and a glass substrate. The results demonstrate that electric field enhancements of >15 relative to the incident fields can be achieved near a silicon tip, implying intensity enhancements of several orders of magnitude. This enhancement arises both from the antenna effect of the elongated probe and from a proximity effect when the probe is near the substrate surface and its image dipoles play a role.
Scanning Probe Microscopy
Near-field optics
Nanometrology
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In this review we describe fundamentals of scanning near-field optical microscopy with aperture probes. After the discussion of instrumentation and probe fabrication, aspects of light propagation in metal-coated, tapered optical fibers are considered. This includes transmission properties and field distributions in the vicinity of subwavelength apertures. Furthermore, the near-field optical image formation mechanism is analyzed with special emphasis on potential sources of artifacts. To underline the prospects of the technique, selected applications including amplitude and phase contrast imaging, fluorescence imaging, and Raman spectroscopy, as well as near-field optical desorption, are presented. These examples demonstrate that scanning near-field optical microscopy is no longer an exotic method but has matured into a valuable tool.
Aperture (computer memory)
Near-field optics
Scanning Probe Microscopy
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