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Near-field scanning optical microscope
Near-field scanning optical microscopy (NSOM/SNOM) is a microscopy technique for nanostructure investigation that breaks the far field resolution limit by exploiting the properties of evanescent waves. This is done by placing the detector very close (distance much smaller than wavelength λ) to the specimen surface. This allows for the surface inspection with high spatial, spectral and temporal resolving power. With this technique, the resolution of the image is limited by the size of the detector aperture and not by the wavelength of the illuminating light. In particular, lateral resolution of 20 nm and vertical resolution of 2–5 nm have been demonstrated. As in optical microscopy, the contrast mechanism can be easily adapted to study different properties, such as refractive index, chemical structure and local stress. Dynamic properties can also be studied at a sub-wavelength scale using this technique.
NSOM/SNOM is a form of scanning probe microscopy.
==History==
Edward Hutchinson Synge, a scientist, is given credit for conceiving and developing the idea for an imaging instrument that would image by exciting and collecting diffraction in the near field. His original idea, proposed in 1928, was based upon the usage of intense nearly planar light from an arc under pressure behind a thin, opaque metal film with a small orifice of about 100 nm. The orifice was to remain within 100 nm of the surface, and information was to be collected by point-by-point scanning. He foresaw the illumination and the detector movement being the biggest technical difficulties. John A. O'Keefe also developed similar theories in 1956. He thought the moving of the pinhole or the detector when it is so close to the sample would be the most likely issue that could prevent the realization of such an instrument.〔(【引用サイトリンク】title=Brief History and Simple Description of NSOM/SNOM Technology )〕 It was Ash and Nicholls who, in 1972, first broke the Abbe’s diffraction limit using radiation with wavelength of 3 cm. A line grating was resolved with a resolution of λ0/60. A decade later, a patent on an ''optical'' near-field microscope was filed by Pohl, followed in 1984 by the first paper that used visible radiation for near field scanning. The near-field optical (NFO) microscope involved a subwavelength aperture at the apex of a metal coated sharply pointed transparent tip, and a feedback mechanism to maintain a constant distance of a few nanometers between the sample and the probe. Lewis et al. were also aware of the potential of an NFO microscope at this time. They reported first results in 1986 confirming super-resolution. In both experiments, details below 50 nm (about λ0/10) in size could be recognized.
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