Guilin Sun
Ansys Employee

The source is the exitation source. Depending on the device condifguration, it can be mode source from the regular fiber, or D-shaped fiber, or that region that can create SPR.  Please find some reference papers and check what they use, and what is the dimension of the whole device. FDTD may not be very friendly if the dimension is in mm or longer, with cross section in hundreds of microns.