FDTD does not provide a single material model that includes all nonlinear processes that occur in Silicon. There are a number of available material models (eg. Kerr, chi, etc) that users may choose from depending on the specific process of interest. It is important to recognize that nonlinear simulations tend to be far more complicated than linear simulations. Finally, it is possible for users to write their own material models using the flexible material plugin framework, but this is a very advanced feature that requires users to have a very high level of knowledge about the FDTD method. We do not provide support for developing custom material plugins.