Abstract: Periodic gratings with different shapes are designed to study spectral reflectivity in normal direction using finite-difference time-domain method.It shows that very low reflectivity in wavelengths can be found with proper grating structures and even zero-reflectance can be found.Influences of depth and slot width of triangle gratings on reflectance are investigated.Parameters for a triangle grating with very low reflectivity in wave bands of 2.75-3.23 μm and 3.84-4.88 μm are found.

Key words: periodic grating, zero-reflectance, reflectivity, FDTD method