I have two questions regarding calculating extinction.
1- i have calculated the sacttering, abs and extinction cross sections. i want to divide them by the area. i am confused about the area. should i include all area in FDTD region or only the part of metal rings

second question:

this is my scattering and abs. crossections curves where i should have only three peaks where they both coincide
for scattering cross section curve mutiple resonance occurs which might be due to instability in structure.
is their away to enhance it.

  1. It will really depend on how you want to define the cross section. The power injected by a plane wave returned by the sourcepower command is that power in watt injected in the simulation region - unit cell. So it does not sound like crazy to use the area of the unit cell but again it will be really up to you (or depends on the definition of the cross section of the reference that you are trying to reproduce the results). Note, the sourceintensity command will be useful if you decided to normalize to the area of the unit cell.

  2. It will be hard for me to make any specific comments without looking at your file. If you dont mind uploading the file here, I am happy to take a look at it.

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Ok, mai, regarding the cross section area used for normalization.I believe the cross section area of the ring makes much sense.
Did you try to increase the simulation time? Also, it could be due to over-meshing.