Topological transitions and topological beam splitters in gyromagnetic metamaterials

Topological transitions unveil a fundamentally novel pathway of research, revealing captivating optical phenomena in electromagnetic metamaterials. In this work, we give comprehensive pictures of topological phase diagrams and topological transitions in the gyromagnetic metamaterials (GMs). We comprehensively demonstrate the topology of all types of equifrequency surfaces of the GMs. The Weyl points and the nodal line can coexist in the GMs, and they are the critical points in the topological transitions. The localized Fermi arc surface states exist at the boundary between the vacuum state and GMs owing to the bulk -edge correspondence of the material system. Full -wave simulations reveal that topological surface waves can smoothly transmit forward around the square defect without reflection or scattering. Remarkably, different types of topological beam splitters are demonstrated utilizing the topological surface waves in the GMs. We theoretically demonstrate that the physical mechanism of achieving the topological beam splitters is caused by different group velocity directions of the Fermi arcs surface states of the material system. Moreover, the topological wave division and controllable propagation of the Fermi arc surface states can be achieved by adopting the different boundary configurations and gyromagnetic parameters in the topological beam splitters. Our work could broaden insights into topological wave physics and provide more flexibility for photonic devices in the electromagnetic media.