Realization of topological fast and slow light based on gradient sandwich structure

Topological photonics offers an attractive platform for the propagation of light with extremely high transmission and the enhancement of light-matter interaction, even in the presence of sharp corners and defects. Most topological structures are designed to support only one case of fast or slow light during frequency routing. However, this work simultaneously realized the high transmission of fast light and the rainbow trapped by slow light with gradient sandwich structure. With the shrinking perturbation index m increasing gradually, the proposed structure supports the trapping of light because the group velocity of transverse electric (TE) sandwich topological edge states can be slowed to near-zero areas, but the propagation of light because of its bulk bands overlapping the near-zero areas of transverse magnetic (TM) ones. Our work introduces a new approach to utilizing sandwich topological edge states for building multifunctional topological light optical devices.