Broadband Multiple Topological Rainbows

Topological photonics offers enhanced control over electromagnetic fields by providing a platform for robust trapping and guiding topological states of light. The topological rainbow can separate and distribute different wavelengths of topological photonic states into different positions, but related topological devices have not yet been fully explored. In this work, topological rainbows are realized by inserting a sandwiched gradient structure in the topological waveguide to realize the topological edge states. A robust one-way slow-light coupling state is realized to broaden the frequency range of the formed topological rainbow with the robust transmission. Thus, multiple topological rainbows are realized by combining the topological property with immunity to backscattering and coupling states of the slow light. Accordingly, light can spread, separate, and thus be trapped at different positions in different directions. This work provides a new way to reconfigure the topological rainbow and brings opportunities to study nanophotonic topological devices.