“…The former includes multi-spatial grid method [3], split-field method (SFM) [4][5][6], spectral FDTD method [7], and exponential time differencing method (ETD) [8,9], etc. The later includes multiple unit cell method [10], and sine-cosine method [11], etc. Compared with indirect methods, direct field methods have many advantages: they have the same stability condition and numerical error as those of the conventional FDTD method, and have simple formulations.…”