Guilin Sun
Ansys Employee
Ok, so you want to simulate the infinite/periodic structure with a partially coherent source, right?
If your definition about the partially coherent source is exactly the same as the paper, you may not be able to simulate the infinite structure correctly without approximation: the paper actually simulated limited size of the periodic structure, as shown here

This is because, according to my shallow understanding to the paper, the Partially-Coherent Gaussian Schell-Model Beam has limit size. This is why the authors used TFSF source.
When you want to simulate periodic/infinite structure, it will need a source with infinite size, which can only be available from plane wave, with periodic-type BCs.
As far as I know, partially coherent beam can have different definitions , which can be spatially, and temporally. So once again, please check if the partially coherent source you want to simulate is the same as in the paper, and if you really want to simulate infinite structure with interaction of light everywhere in the structure. If the structure is infinite, but the source has finite size, You may not be able to use periodic-type BCs as they are for infinite structure AND source, without approximation.
Please note that you are exploring new feasibility beyond the normal capacity of FDTD which is pretty much appreciated, but we need to understand the theory behind the simulation carefully.