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Near-fieic

Near-fieic

Sample

Far-fieid

FIGURE 12.2 Schematic diagram showing NSOM principle. A subwavelength-sized aperture confines the laser light and illuminates the sample in close proximity (typically <10 nm) with a depth of h1 (near-field) and h2 (far-field).

Sample

Far-fieid

FIGURE 12.2 Schematic diagram showing NSOM principle. A subwavelength-sized aperture confines the laser light and illuminates the sample in close proximity (typically <10 nm) with a depth of h1 (near-field) and h2 (far-field).

fiber is scanned close to the sample surface. This emanated light passes through an aperture (typically 2-120 nm in diameter, providing a resolution beyond the diffraction limit) with an exponential attenuation away from the tip. The intensity of the evanescent light waves decay exponentially to insignificant levels ~100 nm from the tip. As shown in Figure 12.2, near-field illumination of the sample at a distance h1 is less than the wavelength (l) of the incident light and gives high-resolution images compared to those obtained from the far-field illumination where the distance h2 ยป l and greater than the aperture. Thus, subwavelength details of the image are lost.

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