Periodic boundary condition
A periodic boundary condition simulated periodicity along a direction. Currently, periodicity is only allowed for normal-incident light. In this case, the electromagnetic fields wrap around the $x$ and $y$ directions. See the illustration to the lower right.
Setting periodic boundary conditions
Periodic boundary conditions can be applied along the $x$ and/or $y$ directions in FDTD++, as illustrated by the image to the right. This is accomplished using the BC keyword in the parameters file.
As an example, in order to set periodicity along both $x$ and $y$, one can add the following to the parameters file:
or explicitly set periodicity along these directions:
BC periodic x BC periodic y
The latter case also demonstrates how one may apply periodicity along only one direction.