This paper introduces a new technique which calculates the reflection coefficient for the plane wave incident on planar periodic structures. The method referred to as spectral
finite-difference time-domain (SFDTD) replaces the conventional single-angle incident wave, with a constant transverse wavenumber (CTW) wave. Because the transverse wavenumbers are constant, the fields have no delay in the transverse plane ( plane), and PBC (periodic boundary condition) can be directly implemented in the time domain for both oblique and normal incident waves. The stability criterion for this new FDTD technique is angle-independent and therefore this method works well for incident angles close to grazing as well as normal incident. This shows the efficiency of the method compared to other available FDTD techniques for the same purpose that force a more restricted stability criterion as angles turns to grazing. The validity of this method is verified by comparing the
reflection coefficient calculated by this met