I. Introduction
The finite-difference time-domain (FDTD) method is one of the most popular three-dimensional (3-D) numerical methods in the field of computational electromagnetics, since it can easily address complex geometrical features and inhomogeneous materials. It has been utilized in many applications, such as microwave cavities and circuits, antennas, scattering by general targets, biomedical applications, semiconductors, high-speed electronic circuits, and optical applications [1]–[4]. Furthermore, a wideband frequency response can be obtained in just one computation, by taking a Fourier transform of the FDTD-computed transient fields.