The Electromagnetic Waves, Time Explicit Interface includes so-called absorbing layers, also often referred to as sponge layers. The layers work by combining three techniques: a scaling system, filtering, and simple nonreflecting conditions. For a review of the method see, for example, Ref. 1.

The layers are set up by adding the Absorbing Layer under the Definitions node. This adds a special scaled system. The scaling effectively slows down the propagating waves and ensures that they hit the outer boundary in the normal direction. For the Absorbing Layer domain selection, add an additional Wave Equations feature, mark the Activate check box under the Filter Parameters section, and enter filter parameters. Filtering attenuates and filters out high-frequency components of the wave. Finally, at the outer boundary of the layer add a simple Scattering Boundary Condition condition, which will work well to remove all remaining waves as normal incidence has been ensured.

For the Absorbing Layers select the Type (Cartesian, cylindrical, spherical, or user defined) under the Geometry section. Enter values for the Physical Width and Pole Distance under the Scaling section.

For the layers to work optimally the filter should not be too aggressive. Moreover, the scaled coordinates in the layer domain should also vary smoothly. To inspect the scaled system you can plot the coordinate variables x_absorb_ab1, y_absorb_ab1, and z_absorb_ab1. Using the absorbing layers with the three combined techniques will enable the reduction of spurious reflections by a factor between 100 and 1000 compared to the incident amplitude.