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Light at optical wavelengths, 500 nm for example, cannot be focused to spots much smaller than approximately one micrometer due to diffraction. This diffraction limit prevents optical imaging beyond the micrometer scale. Near-field optical microscopy (NFOM) uses a light source of less than 100-nm diameter to beat the diffraction limit of conventional optical microscopy. The main concept of NFOM is to place the sample very close to the light source, i.e., in the near field, so that the imaging resolution is determined by the diameter of the light source. InstrumentationThe near-field light source is made from a glass capillary that is heated with a CO2 laser and pulled to an atomically-sharp tip. The outside of the capillary is coated with silver for reflectivity except at the very end of the tip. Laser light is focused into the glass capillary and propagates to the tip by internal reflection (as in optical fibers). A small amount of the light leaks out of the tip via the evanescent wave. The near-field source is brought very close to a sample, and the sample is rastered back and forth to produce an image. NFOM can use either absorption or fluorescence as the optical signal.
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