Hi Sam Sung,
By reading Optiwave FDTD introductions I don’t think I can be sure of the answer to to your question, so I’d better not suggest anything when I am not certain. You have indeed corrected it by stating this this being photonic and not electromagnetic, but did not point out that HFSS (which you had formerly mentioned) was only for electromagnetic cases. Ironically, I was not even completely sure about it.
Regards