Near field diffraction

E. Betzig, A. Harootunian, A. Lewis, and M. Isaacson, Near-Field Diffraction by a Slit - Implications for Superresolution Microscopy, Appl. Opt. 25, 1890 (1986)... 

Figure Bl.22.11. Near-field scanning optical microscopy fluorescence image of oxazine molecules dispersed on a PMMA film surface. Each protuberance in this three-dimensional plot corresponds to the detection of a single molecule, the different intensities of those features being due to different orientations of the molecules. Sub-diffraction resolution, in this case on the order of a fraction of a micron, can be achieved by the near-field scaiming arrangement. Spectroscopic characterization of each molecule is also possible. (Reprinted with pennission from [82]. Copyright 1996 American Chemical Society.)...

By deriving or computing the Maxwell equation in the frame of a cylindrical geometry, it is possible to determine the modal structure for any refractive index shape. In this paragraph we are going to give a more intuitive model to determine the number of modes to be propagated. The refractive index profile allows to determine w and the numerical aperture NA = sin (3), as dehned in equation 2. The near held (hber output) and far field (diffracted beam) are related by a Fourier transform relationship Far field = TF(Near field). 

Consequently the outlines of the far field are characteristies of the smallest detail in the near field. Due to the diffraction laws it is possible to link the speckle substructure typical dimension to the numerical aperture ... 

For samples thicker than the depth of field, the images are blurred by out-of-focus fluorescence. Corrections using a computer are possible, but other techniques are generally preferred such as confocal microscopy and two-photon excitation microscopy. It is possible to overcome the optical diffraction limit in near-field scanning optical microscopy (NSOM). 

The resolution of a conventional microcope is limited by the classical phenomena of interference and diffraction. The limit is approximately X/2, X being the wavelength. This limit can be overcome by using a sub-wavelength light source and by placing the sample very close to this source (i.e. in the near field). The relevant domain is near-field optics (as opposed to far-field conventional optics), which has been applied to microscopy, spectroscopy and optical sensors. In particular, nearfield scanning optical microscopy (N SOM) has proved to be a powerful tool in physical, chemical and life sciences (Dunn, 1999). 

The idea of near-field optics to bypass the diffraction limit was described in three visionary papers published by Synge in the period 1928-32. Synge s idea is illus-... 

Mechanistic investigations of gas-solid and solid-solid reactions as well as their proper engineering require identifiable crystal surfaces for atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM) [1,3, 13-15] in combination with X-ray diffraction data, which are the basis of crystal packing analyses [1,3,16-18]. 

Bachelot, R., Gleyzes, R, and Boccara, A. C. 1995. Near-field optical microscope based on local perturbation of a diffraction spot. Opt. Lett. 20 1924-26. 

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]. 

Unfortunately, the diffraction limit does not permit spectral images to be collected from cells unless the near-field advantage is exploited. However, we have mapped mytotic cells using Raman imaging microspectroscopy, and have reported spectral images of mytotic cells in the telophase and metaphase.17... 

To date, a number of chemically selective near-field imaging methods have been demonstrated. Near-field contrast mechanisms that rely on electronic spectroscopy (UV-visible absorption and fluorescence),204 vibrational spectroscopy (IR absorption and Raman spectroscopies), dielectric spectroscopy (microwave dispersion), and nonlinear spectroscopy (second harmonic generation) have all been demonstrated at length scales well below the diffraction limit of light. 

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. 

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