Wideband ultraflat slow light with large group index in a W1 photonic crystal waveguide

We demonstrate that slow light with large group-index, wideband, and low dispersion can be realized in a silicon-on-insulator W1-type photonic crystal waveguide by simply shifting the first two rows of air-holes adjacent to the waveguide to specific directions. Keeping the group index at 46, 60, 86, 111, 151, and 233, respectively, while restricting its variation within a ± 10% range, we accordingly obtain a slow light bandwidth of 9.0 nm, 6.7 nm, 4.6 nm, 3.3 nm, 2.4 nm, and 1.7 nm, respectively. The normalized delay-bandwidth product keeps around 0.25 for all cases. Moreover, we obtain ultraflat slow light with bandwidths over 3.0 nm, 2.4 nm, 1.6 nm, 1.3 nm, 0.93 nm, and 0.6 nm, respectively, where the group index variation is in a range of only ± 0.8%. Numerical simulations are performed, utilizing the 2D plane wave expansion method and the finite-difference time-domain method.