I am trying to understand exactly why we can beat the diffraction limit when using near-field scanning optical microscopy (NSOM). For those who aren't familiar with NSOM, check out this article: http://www.optics.rochester.edu/workgroups/novotny/papers/history4.pdf.

I can't find any resources that explicitly explain why working in the near-field allows us to circumnavigate the diffraction limit, and I like to have things explained step-by-step. I understand that, in the near-field, we are at a working distance of much less than the wavelength of the light source (and the aperture is also much smaller than the wavelength of the light). Thus, the path length difference of different rays at the object will not be long enough to introduce construction/destructive interference - the basis for diffraction. I know that evanescent waves also contribute to greater resolution, but I don't fully understand how or why. I was hoping someone could explain this to me.



In case you are still looking for the answer:

  • Diffraction is one physical phenomenon, but it can be described in different ways.
  • One approximation is the Fraunhofer limit (valid in far field), another is the Fresnel limit (in near field).
  • Abbe's diffraction limit theory assumes the Fraunhofer assumption.
  • Hence a way to break the diffraction limit, is to image in the near field where the Fraunhofer limit is invalid.

Take a look at any derivation of the Airy disk, such as in the article "The theory of diffraction-limited resolution in microparticle image velocimetry". It always starts with the assumption of Fraunhofer diffraction.


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