Perfectly matched layers and transformation optics

The perfectly matched layer (PML) is a very versatile and effective absorbing boundary condition. The PML was first introduced by J.P. Berenger (1994) for the finite- difference time-domain (FDTD) method. The general purpose of the PML is to produce reflectionless absorption of waves in the continuum limit. Due to its versatility, the PML was later extended to the finite-element and finite-volume methods. In this paper, we revisit the PML using the concept of complex coordinates and briefly discuss the link between PML and the nascent field of "transformation optics".