Geometrical Parameter Optimization and Dispersion Engineering in Slow‐Light Photonic Crystal Waveguides With Ring‐Shaped Holes

ABSTRACT This work investigates flat‐band slow‐light performance in photonic crystal waveguides featuring ring‐shaped holes. Using Finite Element Method based numerical optimization, we show that precise tuning of the inner radii and spatial arrangement of the first two rows of holes strongly governs the dispersion profile. For the first time, a combined optimization strategy employing independent tuning of both ring dimensions and bi‐directional positional shifts in the first two rows is demonstrated to enhance slow‐light performance in RPCWs. The optimized designs achieve ultra‐low dispersion across 16–31 nm bandwidths, with group indices of 11.2–14.85. A best‐performing structure delivers a normalized delay–bandwidth product of 0.264 and group velocity dispersion ≈10 5 ps²/km, representing a clear improvement over previously reported ring‐hole PCW designs. These results demonstrate a robust pathway for engineering high‐performance slow‐light waveguides suitable for compact delay lines and enhanced nonlinear photonic devices.