Far-field and near-field analysis of finite difference time domain method for harmonic diffractive elements

The far-field analysis of the finite difference time domain (FDTD) method has been presented to ascertain the valid range of the scalar diffraction theory (SDT) for periodic harmonic diffractive optical elements with continuous surface profile. And the near-field analysis was used to ascertain the source of error. The valid range of the scalar diffraction analysis is then determined as a function of the grating period-to-wavelength ratio, period-to-depth ratio, and angle of incidence. The evaluation of diffraction efficiency can help to choose the proper method to design and optimizing harmonic diffractive elements with the advantages of SDT's simplicity and FDTD's accuracy.