Topological photonic crystal provides a robust platform for nanophotonic devices. However, few reports have been found to realize multiple frequency routing based on topological photonic states, which have restricted further applications in the field of nanophotonic devices. Here, for the first time, to the best of our knowledge, we propose an efficient method to realize a topological rainbow based on graded dielectric topological photonic crystals, which are constructed by changing the degree of lattice contraction and expansion. The topological edge states of different frequencies are separated and trapped at different positions. The all-dielectric planar nanostructures of graded topological photonic crystals are low-loss, robust, and easy for integration. This Letter plays a key role in the use of robust nanophotonic wavelength routers, optical storage, and optical buffers.

Photonic-plasmonic hybrid microcavities, which possess a higher figure of merit Q/V (the ratio of quality factor to mode volume) than that of pure photonic microcavities or pure plasmonic nano-antennas, play key roles in enhancing light–matter interaction. In this review, we summarize the typical photonic-plasmonic hybrid microcavities, such as photonic crystal microcavities combined with plasmonic nano-antenna, whispering gallery mode microcavities combined with plasmonic nano-antenna, and Fabry–Perot microcavities with plasmonic nano-antenna. The physics and applications of each hybrid photonic-plasmonic system are illustrated. The recent developments of topological photonic crystal microcavities and topological hybrid nano-cavities are also introduced, which demonstrates that topological microcavities can provide a robust platform for the realization of nanophotonic devices. This review can bring comprehensive physical insights of the hybrid system, and reveal that the hybrid system is a good platform for realizing strong light–matter interaction.