Does this structure have higher order diffraction? please check the diffraction angles. Most likely it is due to PML refelction. Plese
1: use coarser mesh such as mesh accuracy 2;
2: use steep angle PML, with 32 or 64 layers
If the simulation termonates around 100%, please increase the simulation time to be 100 times longer. If it terminates before 95%, you may also need to reduce the autoshutoff min at FDTD/Advanced options
Usually shortening the simulation time is not a good choice, since the DFT result needs the time signal decaying completely.
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