Why absorption cross section is far field phenomenon, as we calculate it over desired volume of a nano-structure ?
Luckily it is the same. Take a spherical nanoparticle. In dipole approximation the field outside of it is bj_1(k r)cos theta, where b is unknown. Far from the nanoparticle it is b e^(i kr)/r cos theta. We determine b at the surface where this term is small. And we use it far away where near field is small. Yet, it is the same value.
Optical Cross section: The apparent geometrical size of an object which would absorb the incoming field. Although the nanoparticle is 5 nm but abs. cross section can be 5 times its geometrical size and all this depends on the nature of material, size, shape, environement surrounding it.
Waseem Raja,
You talk of nanostructure and of far field phenomenon; but, you do not specify the frequencies of interest.
You can see if the attached file and/or the references that you find there can help you.
Best Regards
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