Hello Kumar,
OptiFDTD does provide the means for modelling some non-linearity effects, specifically 2nd order and Kerr (3rd order). We would need to know more about the nature of your simulation to provide more assistance.
1. When you refer to a doped crystal are you referring to doping the material crystal (i.e. doped silicon) or do you mean you are back filling the holes with a second material? Also if it is a doping of your device layer what are you doping it with?
2. What nonlinear effect are you using for this particular device?
Regards,
Scott