Guilin Sun
Ansys Employee
Please note that in order to use symmetry BCs, not only the structure has the symmetry, but also the source polarization must have symmetry, which determines whether it is "Symmetrical" or "anti-Symmetrical" BCs to use.
In your case, since the plane wave is incident at an angle in xz plane, so in x boundaries it has lost the symmetry. You should not use symmetry BCs in such cases.
In order to use symmetric BCs, the polarization direction must in the plane of symmetry. Please refer this: Symmetric and anti-symmetric BCs in FDTD and MODE
Please note that for periodic structures+plane wave, in the case of normal incidence, periodic BCs are always correct, and symmetry BCs also work if the structure has axial symmetry. In the case of angled incidence, caution should be taken:
in the plane of incidence (in your current case it is xz plane), no symmetry BCs can be used; However in the other plane symmetry BCs can be applied since it does not see any angle.
While no symmetry BC can be applied in the angled plane, the periodic BCs can also NOT be a choice: You have to use Bloch BCs: Bloch boundary conditions in FDTD and MODE
In case the simulation is broadband, you can either sweep the wavelength, or use BFAST plane wave: Broadband Fixed Angle Source Technique (BFAST)