This lecture presents the perfectly matched layer (PML) absorbing boundary condition (ABC) used to simulate free space when solving the Maxwell equations with such finite methods as the finite difference time domain (FDTD) method or the finite element method. The frequency domain and the time domain equations are derived for the different forms of PML media, namely the split PML, the CPML, the NPML, and the uniaxial PML, in the cases of PMLs matched to isotropic, anisotropic, and dispersive media. The implementation of the PML ABC in the FDTD method is presented in detail. Propagation and reflection of waves in the discretized FDTD space are derived and discussed, with a special emphasis on the problem of evanescent waves. The optimization of the PML ABC is addressed in two typical applications of the FDTD method: first, wave-structure interaction problems, and secondly, waveguide problems. Finally, a review of the literature on the application of the PML ABC to other numerical techniques of electromagnetics and to other partial differential equations of physics is provided. In addition, a software package for computing the actual reflection from a FDTD-PML is provided. It is available here.

Introduction.- The Requirements for the Simulation of Free Space and a Review of Existing Absorbing Boundary Conditions.- The Two-Dimensional Perfectly Matched Layer.- Generalizations and Interpretations of the Perfectly Matched Layer.- Time Domain Equations for the PML Medium.- The PML ABC for the FDTD Method.- Optmization of the PML ABC in Wave-Structure Interaction and Waveguide Problems.- Some Extensions of the PML ABC.