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  Scanning Near-Field Optical Microscopy
   - Development and Applications to Polymer Science

Near-Field Optics: [Polymer Thin Film] [Structure of Polymer Gel] [High-Resolution Patterning]
                          [Single Molecule Spectroscopy] [Development of Novel Near-Field Spectroscopy]

The spatial resolution of optical microscopy is theoretically limited to about half of the wavelength (i.e. about 300 nm in the visible range). This limitation is called the diffraction limit. Scanning near-field optical microscopy (SNOM) have been developed as the "optical" microscopy to overcome the barrior of the wavelength. As shown in the right figure, SNOM uses a sharpend optical fiber having an aperture much smaller than the wavelength of light (typically <100nm). The near-field light emanating from such the small aperture does not propagates to far-field but is confined just in the vicinity of the probe end, which allows one to illuminate the local area beyond the diffraction limit of light.

The most advantage of SNOM is that it uses the "light" as the probe. SNOM enbles one to obtain a high-resolution micrograph with chemical contrast through spectroscopic measurements. In our laboratory, nanometric structures of various polymer systems have been investigated by the near-field spectroscopy since 1995. As well as the applications of SNOM, we study also on the development of novel near-field microscopy systems.

1. Phase Separation Structure of Polymer Monolayer

Phase separation structures of binary polymer blend monolayers was studied by SNOM. The selective imaging of each phase was preformed by the fluorescence labeling of both components with pyrene and perylene dyes. The energy transfer emission was obtained to reveal the structure of the phase interface. 

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2. Internal Structure of Polymer Gel

Inhomogeneous structures of poly(methyl methacrylate) gels were visualized. The time-resolved SNOM measurement revealed the hierarchy structure on the length scale from 10 to 1000 nm. 

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3. High Resolution Patterning on Polymer Matrix by Optical Near-Field

The near-field light confined at the probe end is able to bring about photo-chemical reaction in a nanometric area. SNOM can be used not only as an optical imaging tool but also a photo-pattering device. The right image indicates the Japanese character meaning "near-field" written on a polymer monolayer. The line width was 170 nm, which was beyond the diffraction limit of light. 

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4. Single Molecular Spectroscopy by SNOM

Extremely weak fluorescence from a single molecule can be detected by a recent sensitive photo-detector. The figures depict the SNOM images of individual Rhodamine 6G molecules (the scale bars indicate 100 nm). The size of the fluorescence spot in the right image indicates that the spatial resolution of the system was ca. 20 nm. In the left image, the signal intensity for the arrowed molecule suddenly decreased to the background level while scanning. The abrupt photo-bleaching indicates that the observed fluorescence spots were attributed to the individual dye molecules. Moreover, the molecules were observed as double-lobe-shaped fluorescence spots just like "coffee beans." This indicates that the direction of the electronic transion moment of the dye molecule was perpendicular to the substrate surface.
Recently, our interest is focused on the conformation of single polymer chains.

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5. Development of Novel Near-Field Microscopy

There is strong demand to extend variety of observable chemicals. Large part of chemical species is excited by deep ultra-violet (DUV) light and emits fluorescence. Therefore, it is expected that SNOM using DUV light source can directly "see" various kinds of chemical compounds with a high spatial resolution. We have, for the first time, developed the DUV-SNOM imaging system. 

Selected Publications
1. H. Aoki, S. Ito, J. Phys. Chem. B, 105, 4558 (2001).
2. H. Aoki, Y. Kunai, S. Ito, H. Yamada, K. Matsushige, Appl. Surf. Sci., 188, 534 (2002).
3. H. Aoki, S. Ito, Thin Solid Films, 449, 226-230 (2004).
4. J. Yang, R. Sekine, H. Aoki, S. Ito, Macromolecules, 40, 7573-7580 (2007).
5. T. Ube, H. Aoki, S. Ito, J. Horinaka, T. Takigawa, Polymer, 48, 6221-6225 (2007).

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