Perspective on the topological rainbow.

Topological photonics provides a robust platform for the study of nanophotonic devices. The topological rainbow can be used to separate, slow, and trap topological photonic states of different frequencies at different positions. Although numerous reports have investigated the construction of traditional rainbow devices, limited methods have been proposed to realize topological rainbow phenomena and devices. In this Perspective, we provide an overview of the basic concept and mechanisms of rainbow trapping. A topological rainbow is discussed in terms of the implementation of synthetic dimensions. Additionally, recent advances of the topological rainbow are presented for elastic and acoustic waves. We introduce current physical methods of realizing the topological rainbow and discuss potential applications in physics and engineering. [ABSTRACT FROM AUTHOR]