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In FDTD the peak location may also depend on the simulation time as it accumulates the power for each round trip of the light. You can use smaller autoshutoff min with sufficiently long simulation time. you can use check point for continuing simulation with smaller autoshut off min:https://optics.ansys.com/hc/en-us/articles/360034382534-FDTD-solver-Simulation-Object
When you say PML does not affect the result, it might be true for your specific mesh. When mesh becomes finer please make sure PML has sufficient thickness.
RCWA converging test is a little tricky. Please enable "use tangent vector field". Make sure the mesh refinement uses the same as FDTD's. Use larger difference of k vectors, say 500,1000,1500 to see the trends. When too many k vector values it can be slow You know you are doing the converging test, not direct simulation with resonable result. RCWA uses Fourier decomposition of the complex fields. So if there is discontinuous, it will need many Fourier waves. Please refer to Wiki on RCWA to get more info, eg, efficency, memory and time.
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