Guilin Sun
Ansys Employee

Do you mean the material index with perturbation, or the effective index of the mode?

The conversion theory is here

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.