说明：  一維光子晶體，用平面波展開法求能帶圖，其中傅立葉係數用FFT所求出(不同於直接帶入解析解作法)，code很簡單
(One-dimensional photonic crystal with a plane wave expansion method for the energy band diagram, where the Fourier coefficients are calculated using FFT (as opposed to directly into the analytical solutions practice), the code is very simple)