Near-diffraction-limited resolution

In total internal reflection microscopy, the light is confined only to the z-direction, resulting in the diffraction-limited resolution in the xy-plane. In the last two decades a novel optical microscopy has been developed aiming at high spatial resolution by the three-dimensionally localized optical field, optical near-field . Details on the structural analysis using the near-field microscopy is described in the next section. 

In the science of radio astronomy continuum radiation and absorption or emission lines due to celestial phenomena are observed in the frequency region of 1—300 GHz (300 mm > A > 1mm). The absorption bands due to the terrestrial atmovsphere, as illustrated in Fig. 7.24, will clearly place limitations on such measmements. The diffraction-limited resolution for a telescope is determined by the ratio A/d, where d is the diameter of the telescope. In a comparison with optical telescopes it can be noted that a radio telescope has 10" to 10 times worse angular resolution for a given value of d. For practical reasons a telescope diameter of the order of 100 m is an upper limit, especially since a surface precision of about A/20 must be maintained. Examples of radio telescopes are the 100 ni telescope near Bomi in Germany, which is used in the frequency region 1—45 GHz and the 20 m Onsala telescope in Sweden, which can be used for frequencies up to 120 GHz (A = 2.5 mm). A diagram of the latter telescope is shown in Fig. 7.28. 

X-Ray Microscopy. Because of the short wavelength of x-rays, they have, for nearly 100 years, held out the hope of being utilized in order to significantly lower the diffraction limit of resolution when visible light is used. The difficulties of focusing x-rays and the relative weakness of x-ray sources have, until recently, fmstrated efforts to teach that goal (25). 

Using a visible light probe NSOM is the eadiest of the probe scopes, at least in conception, and is another apparent exception to the diffraction-liinited resolution rule, in that NSOM illuminates an object with a beam of visible light smaller than the diffraction limit. The resolution then is limited only by the size of that beam. To achieve this, light issuing from a very tiny aperture at the end of a glass capillary scans a very near sample. The tip must be located on the order of X/2 from that surface. Resolution in the range of 10—20 nm has been achieved (31). 

SNOM combines the optical contrast with a high lateral resolution of SPMs [55,56]. Scanning a surface with a sharp optical fibre tip within the range of the optical near field makes it possible to overcome the optical diffraction limit that restricts the resolution of conventional optical microscopy. Moreover, the SNOM probe operates at a finite distance from the surface, so that damage and distortion of delicate samples can be eliminated. The drawback of SNOM compared to other SPM methods is its relatively low resolution - around tens of nanometers [62,63]. 

There is a need to improve probe geometries for high-resolution chemical imaging beyond the diffraction limit. This includes design (theory) and realization (reproducibility, robustness, mass production) of controlled geometry near-field optics. 

Near-Field Scanning Optical Microscopy (NSOM) is a technique which enables users to work with standard optical tools integrated with scanning probe microscopy (SPM). The integration of SPM and certain optical methods allows for the collection of optical information at resolutions well beyond the diffraction limit. 

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