Polarization-independent one-way transmission by silicon annular photonic crystal antireflection structures

A triangular annular PC, which is a half of square-lattice silicon annular PC divided along diagonal line, is constructed. Band structures of the annular PC are investigated through the plane wave expansion method, and the directional bandgap mismatch for both TE modes and TM modes is presented. The finite-different time-domain simulations are used to calculate the TE-like and TM-like light transmission spectra, contrast ratio and field distributions. Though the triangular annular PC displays polarization-independent one-way transmission characteristic, the forward transmissivity (20%) is too low. Antireflection coating layer is introduced to increase forward transmissivity. Polarization-independent one-way transmission performance is improved by two antireflection coating layers added on the hypotenuse of the triangular annular PC, and the forward transmissivity for two polarizations increases to 46%.