Boundary-induced three-dimensional one-way bulk transport in gyromagnetic systems

In three-dimensional (3D) topological physics, the bulk properties of a material fundamentally determine the existence and characteristics of its boundary states. However, these boundary-state fields are usually localized near the interface of the bulk, thereby losing their high-throughput information processing capabilities and the controllability of their mode width. Here, in contrast to this conventional perspective, we unveil theoretically a class of one-way topological chiral extended states (CESs) that extend throughout the 3D photonic Weyl waveguide. These states arise solely from the modulation effect of the waveguide boundaries and can persist over a broad range of waveguide widths. However, in the designed 3D gyromagnetic photonic crystal Weyl waveguide, spectrally isolated and practically accessible one-way operation is subject to a finite-width limitation, with our relative-bandwidth analysis indicating an upper bound of d / L ≈ 35. We demonstrate that the CES physically originates from the formation of odd or even modes, which create nonzero topological charge Weyl points. The CES showcases two-dimensional (2D) discrete momenta and has a strong anti-scattering one-way transmission characteristic for 3D scatterers. Moreover, based on the pattern-mismatch responses of CES with different symmetries, we design a 3D one-way topological photonic filter with tunable bandwidth. Our work unveils boundaries as a new degree of freedom for controlling bulk Weyl quasiparticles, establishing a foundation for the development of 3D one-way high-throughput, robust energy-transport materials and devices.