Guilin Sun
Ansys Employee

What kind of nonlinearity is this? if it creates new frequencies, then even the Bloch BCs cannot be used, as Bloch BC works for a single wavelength. 
Both BFAST and Bloch BCs only work for periodic structure.
If it is not periodic, you could try to use broadband Gaussian beam. Or you may try to tilt the structure with normal incidence.