The
operational principle behind near-field optical
imaging involves illuminating a specimen through a sub-wavelength
sized aperture whilst keeping the specimen within the near-field regime of
the source. Broadly speaking, if the aperture-specimen separation is kept
roughly less than half the diameter of the aperture, the source does not have
the opportunity to diffract before it interacts with the sample and the resolution of the system
is determined by the aperture diameter as oppose to the wavelength
of light used. An image is built up by raster-scanning the aperture across
the sample and recording the optical
response of the specimen through a conventional far-field microscope
objective. (As opposed to conventional optical
microscopy or 'far-field optical
microscopy').
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