Do you mean the material index with perturbation, or the effective index of the mode?
The conversion theory is here https://optics.ansys.com/hc/en-us/articles/360034382494-Charge-distribution-to-change-in-refractive-index-theory
So if the charges are uniform, you can get the perturbated material complex refractive index:
If the np density is simulated from CHARGE, it is not uniform then you cannot import or get the refractive index perturbation.
However when the np density is applied to a waveguide, and you use FDE or FEEM you will be able to get the mode effective index, real and imaginary parts.
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