Apply two-level plane wave time domain method in implementing multi-region finite difference time domain method

The finite difference time domain method (FDTD) is widely used to simulate electromagnetic fields in various cases. We adopt multiregion FDTD (MRFDTD). In MRFDTD, ordinary FDTD is applied separately in every region, and interactions among these regions are accounted for by the Kirchhoff integral formula (KIF), which is equivalent to Huygen's principle.