Unidirectional wavelength filtering characteristics of the two-dimensional triangular-lattice photonic crystal structures with elliptical defects

• Unidirectional wavelength filters based on elliptical defects are obtained.
• Unidirectional filters towards two opposite directions are acquired.
• Bidirectional filtering is also achieved through the same structure.

The unidirectional wavelength filtering characters of the two-dimensional triangular-lattice photonic crystal structures consisting of two waveguides and an elliptic defect are theoretically studied by the finite-difference time-domain method. Through designing the coupling regions between the elliptical defect and the adjacent waveguides, the unidirectional wavelength filtering is achieved owing to the modes’ match and mismatch between the elliptical defect and waveguides, which converts the incident fundamental even-symmetric modes to the higher order odd modes. Based on the abundant defective modes with different symmetries supported by the elliptical defect, this kind of wavelength filters can allow the unidirectional light propagation with inverse forward directions, and the bidirectional propagation through the same structure at different wavelengths.