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Near-field optics-- how they workObjects that are very small relative to the wavelength of light scatter and diffract light at large angles from the viewing axis. In order to see and fully resolve such objects, the lens must collect all of the scattered light information from them, so that it can form an image at the focus of the lens by a process known as "interference". This is the optical principle underlying microscopes, as Ernst Abbé discovered over a century ago. The smaller an object is relative to the wavelength of light (which is about 0.5 microns for visible light), the larger the angle of the diffracted light. One can make that angle smaller by using light with a shorter wavelength (i.e. more blue), or use a lens with a larger angular collection area (referred to as numerical aperture or NA). Ultimately, as the object gets even smaller, the light is diffracted into such a large angle that it can no longer leave the object-- the angle has an "imaginary sine"-- and the light cannot propagate, or travel to, the lens. The light exists as a bound "evanescent field," so-called because the intensity of the light decays very quickly (exponentially) with distance from the object. Therefore, in order to see a very small object, you have to be very close to it -- hence, "near-field"--, a conclusion which seems obvious, though the underlying physical optics principle is subtle. NANOPTEK has developed technology that allows such proximity to the object without using scanning probes, and further utilizes the exponential decay of the light field to obtain sub-nanometer quantifiable vertical resolution--better than SEM!
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