Fast-light propagation in a zero-index photonic crystal waveguide with photonic bandgap claddings

Abstract We demonstrated zero-index (ZI) photonic crystal (PhC) waveguides (ZIWs) operating at the Dirac point embedded between photonic bandgap (PBG) claddings, which improved fast-light propagation characteristics in the terahertz (THz) regime. The cross-shaped silicon-based waveguide supports a Dirac-like cone at the Brillouin zone center, enabling zero-effective refractive index behavior. By including PBG cladding, extending from circular and triangular to perturbed configurations, our ZIWs efficiently improved mode confinement through strong mode coupling, confirmed by supercell band analysis that revealed two guided modes with distinct dispersion characteristics. Group velocity ( v g ), group index ( n g ) calculations of dispersion bands and delay time analysis demonstrated fast-light propagation near the ZI frequency. The highest value we achieved for v g ​is ~ 236 μm/ps, or about 0.79 times the speed of light in free space, which aligned well with the time delay analysis, and corresponding n g values can numerically reach 1.18, showing a promotion of group velocity of 47.4% to 78.7% for the ZIWs at the Dirac point compared with that of conventional PhC waveguides. Numerical simulations confirmed robust field confinement through the broadband transmission spectrum within the ZI region, particularly for geometrically perturbed PBG structures. Furthermore, the insertion loss and phase shift analyses confirmed its superior performance, showing minimal losses (~ 0.015 to 0.02 dB) and smooth dispersion control. These results describe the potential of ZIWs with tailored PBG environments for applications in ultrafast photonic signal transport, especially for meeting the increasing demands of high-speed connections between chips in artificial intelligence computer centers and for on-chip interconnects among different photonic integrated circuits.