Two-gap chiral surface states and dual-band topological switching effects in photonic gyromagnetic continuous media

Recent advancements have revealed that three-dimensional (3D) topological semimetals support a rich spectrum of Fermi arc surface states, providing a versatile platform for investigating novel physical phenomena. Nevertheless, previous investigations into 3D topological systems predominantly concentrated on surface states within a single frequency band, restricting their applicability in multiband scenarios such as topological photonic switching, routing, and beam splitters. In this work, we demonstrate the existence of two-gap chiral Fermi arc surface states in 3D topological gyromagnetic continuous media. By employing rigorous theoretical analysis, we provide a comprehensive depiction of the phase diagrams for multiple 3D band degeneracies in topological gyromagnetic continuous media and prove the critical conditions of the emergence of two-gap chiral surface states. Physically, these two-gap chiral surface states originate from the non-zero gap Chern numbers of different topological frequencies when the system considers the magneto-optical effect. Notably, the photonic Weyl point is the critical point between this type of two-gap chiral and the traditional single-frequency band surface states. Moreover, three-channel, bandwidth-controllable, and dual-band topological switching effects are demonstrated by leveraging the unique properties of two-gap chiral surface states. We further theoretically demonstrate that the physical mechanism of achieving dual-band topological switching effects is caused by the different boundary configurations. This research could provide deeper insights into 3D photonic topological semimetals and enhance the design flexibility of multiband topological functional devices.