volume | PIER Journals

Abstract

Slow-light photonic crystals with group indices exceeding 100 typically require complex defect engineering - line-defect waveguides, coupled resonator optical waveguides (CROWs), or carefully designed heterostructures. These approaches, while effective, introduce fabrication complexity, optical losses, and bandwidth limitations. Recent advances achieving group indices of 50-100 in engineered structures still fall short of theoretical predictions for bulk photonic crystals operating in higher-order bands. However, a critical knowledge gap persists: most higher bands (3-8) largely unexplored. Here, we demonstrate that bulk square lattice photonic crystals operating in higher-order TM bands achieve ultra-slow light (group index 226.5) and extreme anisotropic dispersion (ratio 58.36) - performance comparable to state-of-the-art defect-engineered waveguide structures, achieved in a bulk crystal without any line defects, coupled resonators, or heterostructures, thereby maintaining fabrication simplicity. Through systematic equifrequency contour (EFC) analysis of bands 1-8 using high-density k-space sampling, we establish a quantitative framework revealing that higher bands offer superior slow-light characteristics unattainable in conventionally studied lower bands. Our GaAs/organic semiconductor system achieves this performance with moderate dielectric contrast (ε_r = 6.86), compatible with standard electron-beam lithography fabrication. A systematic parametric sweep of the rod radius (R/a = 0.30-0.50) further reveals that this slow-light performance is structurally robust and tunable by geometry: the Band 4 group index varies by less than 25% across the full sweep; Band 3 sustains a high normalized delay–bandwidth product (NDBP = 0.40-0.48) over a wide fabrication-tolerant range; Band 4 reaches an NDBP optimum of 0.403 at R/a = 0.38; Band 7 uniquely combines NDBP > 0.1 with an anisotropy ratio above 200; Band 8 exhibits extreme anisotropy (ratio up to 1369).

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Dr. Khee Lam Low

Dr. Chia Yuee Lum