Numerical studies of mode gaps and coupling efficiency for line-defect waveguides in two-dimensional photonic crystals

Using conventional waveguides for light coupling and collection we numerically study band structures and transmission spectra for guided modes in line-defect waveguides obtained by removing rows of air holes in a lossless triangular-lattice two-dimensional photonic crystal. The two-dimensional finite-difference time-domain method combined with the effective index method and complemented with the coupled mode approximation is employed to analyze mode-gaps (or mini stopbands) arising from Bragg diffraction of the incident mode into the counterpropagating modes. We also show that more than 97% coupling efficiency between the ridge and the photonic crystal waveguides is achievable by adjustment of the ridge waveguide width.