I do not know why you cannot post some screenshots. From your description it seems you used a plane wave source but used PML BCs. This is wrong since PML will truncate the plane wave causing diffraction.https://optics.ansys.com/hc/en-us/articles/360034382914-Understanding-the-diffracting-option-of-the-plane-wave-source
The correct way is to use periodic BC if it is normal incidence, or Bloch BC it is angled in xy plane since the propagation is along z axis.
You are navigating away from the AIS Discovery experience