Scanning Near-Field Optical Microscopy
Principle of near-field optical microscopy
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 barrier of the wavelength. As shown in the right figure, SNOM uses a sharpened 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 enables one to obtain a high-resolution micrograph with chemical contrast through spectroscopic measurements.
Spatial resolution beyond the diffraction barrier
A single molecule is a good standard sample to examine the sensitivity and resolution of a microscope. These figures shows fluorescence SNOM images of single rhodamine 6G molecules (scale bars indicate 100 nm). The right image indicate that the resolution of SNOM is 20 nm. In the left image, a rhodamine 6G molecule is observed as a two-lobed pattern. This results from the electric field distribution of the optical near-field around the SNOM tip. The direction of the electric field is vertical to the sample plane at the rim of the sub-wavelength aperture, whereas it is horizontal at the center of the aperture. Therefore, a dye molecule with a vertical orientation emits the fluorescence at the aperture rim, resulting in the two-lobed pattern. Thus the SNOM enables the optical measurement at the single molecule level.
Development of novel near-field spectroscopy
SNOM provides various spectroscopic information from a nanometeric area. We have, for the first time, developed the near-field spectroscopy technique in deep ultra-violet (DUV) region. Large part of chemical species is excited by DUV light and emits fluorescence. This method enables the direct observation of various kinds of chemical compounds without staining process. DUV-SNOM would be a versatile method for the structural analysis of polymer/organic materials.