Light and sound are the most ubiquitous forms of waves, associated with a variety of phenomena and physical effects such as rainbows and echoes. Light and sound, both categorized as classical waves, have lately been brought into unexpected connections with exotic topological phases of matter. We are currently witnessing the onset of a second wave of active research into this topic. The past decade has been marked by fundamental advances comprising two-dimensional quantum Hall insulators and quantum spin and valley Hall insulators, whose topological properties are characterized using linear band topology. Here, going beyond these conventional topological systems, we focus on the latest frontiers, including non-Hermitian, nonlinear and non-Abelian topology as well as topological defects, for which the characterization of the topological features goes beyond the standard band-topology language. In addition to an overview of the current state of the art, we also survey future research directions for valuable applications.
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X.Z. and M.-H.L. are supported by the National Natural Science Foundation of China (grant number 12222407) and the National Key R&D Program of China (grant numbers 2018YFA0306200 and 2021YFB3801800). J.C. acknowledges support from the Spanish Ministry of Science and Innovation through a Consolidación Investigadora grant (CNS2022-135706). J.C. also acknowledges discussions with P. San-Jose.
The authors declare no competing interests.
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Zhang, X., Zangeneh-Nejad, F., Chen, ZG. et al. A second wave of topological phenomena in photonics and acoustics. Nature 618, 687–697 (2023). https://doi.org/10.1038/s41586-023-06163-9
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