Topological flatband of edge states for rainbow trapping in Z2 photonic crystals

Topological photonics has provided a new perspective to manipulate the light and inspires the innovation of various nanophotonic devices. In this work, we propose a novel Z₂ topological PhC structure featuring an edge state flatband with extremely low group velocity across the entire Brillouin zone and protected by a large bandgap. By gradually varying the fusion degree of the dielectric cylinders in the unit cells along the topological waveguide, the flatband can be tailored to different frequency range at different locations of the waveguide, resulting in compact rainbow trapping of the topological interface states. This work provides a novel insight to the functional development of on-chip topological devices for optical buffering and frequency routing.