Unidirectional reciprocal wavelength filters based on the square-lattice photonic crystal structures with the rectangular defects

The novel all-optical unidirectional wavelength filters are studied by the finite-difference time-domain method, which are based on the two-dimensional square-lattice photonic crystal with the rectangular defects. Owing to the modes' match and mismatch between the defect and the adjacent waveguides, the unidirectional propagation of the fundamental-mode light beam resonant at a certain frequency is obtained. Through merely altering the coupling region between the defect and the input waveguide, a unidirectional dual-branch waveguide filter is designed. This kind of devices has both the abilities of wavelength filtering and unidirectional light propagation, and may be potentially applied in the future all-optical complex integrated circuits.