Tunable topological slow-light in gyromagnetic photonic crystal waveguides with unified magnetic field.

We have proposed a tunable topological slow-light in a photonic crystal (PC) waveguide with unified magnetic field. The waveguide is constructed by bringing close two gyromagnetic photonic crystals (GPCs) with different structural parameters and introducing a row of Al2O3 rods as the coupling layer. The two GPCs are applied with a unified external static magnetic field (ESMF) instead of two opposite ESMFs. Such waveguide supports a slow-light state originated from the coupling effect of two one-way edge states on both sides of the waveguide. By simply changing the strength of ESMF, one can achieve a tunable slow-light state with large normalized delay-bandwidth product (NDBP) (0.36< NDBP <0.84). Based on these excellent properties, we further design an optical delayer with a compact structure and expansibility simultaneously. This unique topological slow-light state with simple unified magnetic condition, high maneuverability and strong immunity to defects holds promise for many fields such as signal processing, optical modulation, and the design of various slow-light devices.