Coupling between opposite-parity modes in parallel photonic crystal waveguides and its application in unidirectional light transmission

Directional coupling between the even- and odd-parity modes of two parallel dissimilar linear defect waveguides in a square photonic crystal of cylindrical air holes in dielectric background is numerically demonstrated. Projected band-structure computations through the plane-wave expansion method reveal that high-efficiency coupling can be achieved in a frequency range of approximately 9 % extent around the central frequency. Coupling occurs if one row of spacing is maintained between the waveguides supporting even and odd modes, which are composed of annular air holes with outer radii equal to the photonic crystal's scatterer radii and inner radii of 0.19 and 0.44 periods, respectively. Extinction ratio for coupling from the even to odd mode at the central frequency is 4.0 dB. Coupling length calculated through finite-difference time-domain simulations is approximately 25 periods at the central frequency, in agreement with the estimation through band diagram. Unidirectional light transmission is also demonstrated through finite-difference time-domain simulations, provided that waveguide and coupling lengths are equal. Forward and reverse transmittances of 71 and 0.3 %, respectively, are achieved at the central operation frequency in a 25-period system.