A simple definition might help to understand the diffraction pattern in an aperiodic nano-micro structured surface.
In near field (Fresnel) the wave fronts are considered close to spherical whereas in the far field the wave fronts are esssentially planar (Fraunhofer)
In near field (Fresnel) the wave fronts are considered close to spherical whereas in the far field the wave fronts are esssentially planar (Fraunhofer)
Above answer is brief and correct, the following link explains it in details:
http://www.electronicdesign.com/energy/what-s-difference-between-em-near-field-and-far-field
For a beam focused at infinity, the far-field region involves Fraunhofer region . In the far field, the relationship between the electric field component E and the magnetic component H is that characteristic of any freely propagating wave, where (in units where c = 1) E and H have equal magnitudes at any point in space.
On the contrary, the diffraction pattern in the near field typically differs significantly from that observed at infinity and varies with distance from the source. In the near field, the relationship between E and H becomes very complex. Also, unlike the far field where electromagnetic waves are usually characterized by a single polarization type (horizontal, vertical, circular, or elliptical), all four polarization types can be present in the near field.
For more details see: http://www.hep.manchester.ac.uk/u/xiaguo/waveoptics/Fresnel%20diffraction.pdf
Thank you for the insights & the links everyone!
Rajib Biswas Thank you for including diffraction pattern in your answer. I would like to get more precise at what I am looking for and anticipate to receive your thoughts on it:
I have a 2 um deep semi-spherical cavity with nanometric roughness inside it. How can I justify the locations where near field or far field effects occur? Could it be that inside the cavity and between the adjacent nanometric roughness we have the near field effects and in whole there is a combination of the near field and the far field effects?
I am sorry if the question might seem simple, I am pretty new in optics and explaining the diffraction pattern in this case is quite difficult for me :)
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