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Abstract
The many proposed designs for metamaterial structures and artificial composites have been triggered by some of the specific applications for which metamaterials have been most strongly promoted, such as superresolution imaging or cloaking. Image resolution using conventional optics is limited by diffraction. When a lens is used to construct an image, e.g., by means of a camera, fine details of the image disappear. This is because of the fundamental diffraction limit. Scattered light includes evanescent waves and propagating waves. Features smaller than approximately the wavelength of the light are mostly carried by evanescent waves that decay exponentially away from the source. This means that when a far-field image is constructed using a conventional lens, the subwavelength features are lost, as are those propagating waves with k-vectors that do not transport sufficient energy into the lens because of its finite size (numerical aperture). Evanescent waves encode high spatial frequencies originating from the fine details of the scatterer; therefore, if this information is picked up in the near field (i.e., less than a wavelength away) of the specimen, both evanescent and propagating waves can contribute to the reconstruction of an image, leading to improved and possibly subwavelength resolution. This is the general idea behind the scanning near-field optical microscope, which breaks the diffraction limit by means of a sharp fiber tip that is brought into the near-field zone of the specimen in order to pick up the subwavelength information of the evanescent waves.
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CHAPTER 6
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