Complex coordinate system as a generalized absorbing boundary condition

The perfectly matched layer (PML) in Cartesian coordinates is equivalent to solving the wave equation or Maxwell's equations in a complex coordinate system. A closed form solutions that exist in the real coordinate system map to solutions in the complex coordinate system. In the complex coordinate system, the boundaries exist in a complex space, providing absorbing boundary conditions. Hence, this transformation provides a new view of PML in the Cartesian coordinates, clearly showing that a mapping to a complex coordinate system does not induce reflections, explaining why PML works near the corner of a simulation region, and when a dielectric interface, or a metallic surface, extends to the edge of a simulation region.