To model periodic structures with oblique incident waves/scan angles in FDTD, the field transformation method is successfully used to analyze their characteristics. In the field transformation method, Maxwell's equations are Floquet-transformed so that only a single period of infinite periodic structure can be modeled in FDTD by using periodic boundary conditions (PBCs). A new discretization method based on the exponential time differencing (ETD) algorithm is proposed here for the discretization of the modified Maxwell's equations in the periodic FDTD method. This new discretization method provides an alternative way to discretize the modified Maxwell's equations with simpler updating forms that requires less CPU time and memory than the traditional stability factor method (SFM). These two methods have the same numerical accuracy and stability in the periodic FDTD method. Some validation cases are provided showing perfect match between the results of both methods.
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