Guilin Sun
Ansys Employee

When every time it gives different result, it means the result has not converged.

As you said, FDTD is pretty consistent. you may need to spend more time on RCWA convergenging tests. I would not suggest you to use two priods, as the result should not be better than one period if all the parameters are properly set.

Your structure can be considered as 4 different gratings working together, so it might be somewhat challenge. Please do more tests. If you have premium support, please contact us through email. thank you!