PML boundary conditions are designed to absorb all incident light. For best performance, physical structures should extend completely through the PML boundary condition region. The default settings in FDTD will automatically extend any structures which lie on the PML boundary through the PML. In some cases, such as for photonic crystals, it is better to disable this feature and draw the structures in the PML manually.
A typical simulation is shown in the above screenshot. The simulation region outline is shown graphically in orange. When the boundary conditions are set to PML, the simulation region outline has some thickness. This creates three areas, labeled A,B,C in the above figure.
A - Simulation area of interest
This is the region that the user is interested in simulating. All important structures, sources and monitors should be defined in this area. The size of this area is set with the X,Y,Z span property of the simulation region. Monitors only collect data in this area. Monitors and sources larger than this area will be automatically truncated at the boundary.
B - Boundary condition area
Boundary conditions are applied in this region. For most types of boundary conditions (periodic, bloch, metal), this area is very thin (1 mesh cell thick). These boundary conditions do not need a large area to function. PML boundaries are different in that they require a larger area to properly absorb the fields. The size of this area is proportional to the number of PML layers used in the simulation, and inversely proportional to the mesh size.
C - Completely outside the simulation region
Everything in this area will be ignored because it is completely outside of the simulation region. It is ok to create objects in this area, but they will not be included in the simulation. This applies to structures, sources, and monitors.
Extending objects through the boundary condition area
During a simulation, the electromagnetic fields are calculated both within the simulation area (A) and within the boundary condition area (B). Since the fields are still being propagated in (B), it is important that the material properties are also defined here. An interface in this area will act like any other interface and cause reflections. All structures should extend completely through the boundary condition area to minimize these reflections. This is especially important when using many layers of PML. As more layers of PML are added, the boundary condition area will become larger.
The boundary conditions tab of the FDTD region, shown below, contains an option to extend structures through the PML. By default this option is selected.
If the extend structure through PML option is selected, it will extend any structures that touch the inner PML boundary in the direction normal to the boundary. For instance, if the FDTD simulation contains the structure shown in the left image below, then the extend structure through PML option will create extend the structure as depicted on the right. This may not be ideal for all structures. In that case, you can uncheck this option and draw the structure through the PML.
The left figure below shows the correct setup when using substrates and other layers with PML. The layers should extend through the PML in both the X and Y directions. The right figure shows the layers terminated at the inside boundary of the PML. This interface may create an undesired reflection.
If the extend structures through PML option is selected, FDTD will automatically extend the structure in the right figure so that it gives the same results as the left figure.
The left figure below shows the correct setup when using periodic structures with PML. The structures should extend through the PML boundary. The right figure shows the periodic structure only defined within the simulation volume. This effectively creates an impedance mis-match between the PC region and the boundary condition region. The mis-match will reduce the performance of the PML.
Important: FDTD/MODE only meshs the structure as it is drawn on the left if the extend structures through PML checkbox is unchecked. Otherwise, the material which touches the inside PML boundary will be extended straight through the PML. When the 'extend' option is enabled, the structure will be meshed as drawn on the right, since none of the circles touch the inner edge of the PML boundaries.
Note: Sources and monitors will be automatically truncated to the inner simulation region. We recommend defining the sources and monitors to be larger than the simulation region. That way, if you need to make the simulation region larger, you don't need to re-size all the sources and monitors.
Note: Anything completely outside of the orange simulation boundary region has no effect on the simulation. In the photonic crystal example above, the two outermost rings of PC are not necessary. It is the third ring, within the PML boundary region, that is important to minimize reflections.