Guilin Sun
Ansys Employee
From your description, I guess that the 2D photonic crystals are periodic in xy axes. Since it is freestanding, so the structure is symmetric in z. However, the Boundary conditions in Z will determine its band structure property:
When both Zmin and Zmax use "Symmetric", it means such photonic crystal is periodic! eg, you are simulating pure 2D and perhapes layered with air.
the correct settings for the free standing photonic crystal to get its band structure is to use "Symmetric" at Zmin, but keep Zmax as PML. This will give you the band structure with symmetric modes in z. You can also use "Anti-SYmmetric" to get another band structure. Due to several reasons the band structure might not be perfect. So you may need to fine turn some parameters, such as simulation time, "tolerance" to not over sampled or under sampled.
When there is no symmetry in z,such as with a substrate, you are right to use PML in both Z min and Z max. As mentioned above, you will need to modify some settings and/or parameters in order to get better/clean band structure.