Efficient photonic crystal directional couplers

We demonstrate highly efficient and spectrally flat broadband coupling in photonic crystal directional couplers. The result is obtained by use of a novel design with smaller holes between coparallel photonic crystal waveguides for efficient channel-to-channel coupling. The system studied is based on a planar hexagonal photonic crystal lattice of holes made in silicon-on-insulator material. Results from three-dimensional finite-difference time domain modeling are shown to closely match results measured on fabricated samples.