Except for Some Special Directions Non-Spherical Field or Other Broad Class of Fields.What you think kindly share your concept and personal experience
Evanescent waves in general do not propagte to the far field. But, when we consider multiple scattering (in this particular case, an evascent wave may generate a propagating wave), it is possible that there is contribution of evanscent wave in the far field indirectly. Also, it is possible to transport the near field information to far field through different optical technique and then we can say that the far field contains the contribution of the near field.
why Evanescent light waste and don not reach to detector??? and have you know any technique that we brought some of that light to detector or able to analyze it???
Nano-optics deals with optical phenomena on nanometer scale, that is beyond the diffraction limit of light. The spatial confinement that can be achieved for photons is inversely proportional to the spread of wavevector components in the corresponding spatial direction. Such spread occurs in light field that converges towards the focus (behind the lens). Thus, one can obtain some finite spread in the wavevector components for highest possible NA that can be obtained with oil-based microscope objectives. If one would be able to to achieve a wider spread of wavevectors, then light could be focused to the spot smaller than the one dictated by the diffraction limit and one can achieve sub-wavelength resolution...
In 1928 Synge published an article that outlined the concepts of what is currently known as scanning near-field optical microscopy. He stated if one takes an opaque film with a sub-wavelength aperture in it and places it within a sub-wavelength distance from a sample, then the produced illumination spot will not be limited by diffraction but the size of the aperture! A reason for placing the sample so close to the aperture is because the aperture produces an evanescent wave. This is a non-propagating wave whose field decays very fast, within 100 nm or so. Hence, it doesn't contribute to light collected by detector in the far field. However, it can be detected in the near-field with certain microscopy techniques that are sensitive to near-field light.
In modern days this concept is the basis of the near-field scanning optical microscopy (NSOM) or scanning near-field optical microscopy (SNOM). Both abbreviations have been used. The tip can be made in several ways: metal coated tapered fiber that has an aperture 10-100 nm (achieved by so called shadow coating evaporation technique) or micro-fabricated probes (for example, based on standard AFM cantilever technology). These are just examples because the technology is evolving and so do the methods of making probes with sub-wave apertures.
So, in the nutshell this is a combination of AFM with optical probing. One brings such a probe to a close contact with a sample and scans it across the sample. Collection is done in far-field with a diffraction limited optics. As in optical microscopy, various modes of operation are possible: transmission, reflection etc. One can add non-linear effects as contrast mechanism (SHG, for example), fluorescence etc.
To get more info, do a literature search for NSOM, SNOM and you end up with excellent reviews and original articles on the subject. Alternatively, you may look at some books on the topic like
L. Novotny and B. Hecht "Principles of Nano-Optics", Cambridge University Press (2006)
Eds. Anatoly Zayats and David Richards
"Nano-Optics and Near-Field Optical Microscopy"
Artech House, 2009
Ed. Satoshi Kawata
"Near-field Optics and Surface Plasmon Polaritons"
Springer, 1996
- Badges
- Science topic
- Similar topics
- Engineering
- Electrical Engineering