Boundary-Induced Topological Chiral Extended States in Weyl Metamaterial Waveguides

In topological physics, it is commonly understood that the existence of the boundary states of a topological system is inherently dictated by its bulk. A classic example is that the surface Fermi-arc states of a Weyl system are determined by the chiral charges of Weyl points within the bulk. Contrasting with this established perspective, here, we theoretically and experimentally discover a family of topological chiral bulk states extending over photonic Weyl metamaterial waveguides, solely induced by the waveguide boundaries, independently of the waveguide width. Notably, these chiral bulk states showcase discrete momenta and function as tunnels that connect Fermi-arc surface states living in different two-dimensional spaces via a third dimension. Our work offers an alternative mechanism for robust chiral bulk transport of waves and highlights the boundaries as a new degree of freedom to regulate bulk Weyl quasiparticles.