Cladding-free Fermi arc surface states and topological directional couplers in ideal photonic Weyl metamaterials

Abstract

Photons can freely propagate in the vacuum state, so the vacuum is not a trivial insulator, but a conductor for photons. Because of this reason, in topological photonics, the domain wall structures with opposite effective mass terms as a cladding to confine electromagnetic waves have to be adopted to demonstrate the topological edge/surface waves and Fermi arc surface states. In this work, based on the ideal Weyl gyromagnetic metamaterials (GMs), we demonstrate that can be realized the cladding-free Fermi arc surface states with high field localization on the boundary. In the GMs, the ideal Weyl semimetal phase exists due to the dispersionless longitudinal modes. The claddingfree Fermi arc surface states connect the projections of the Weyl points with opposite chirality at the boundary owing to the bulk-edge correspondence of the vacuum-GMs system. Full-wave simulations further demonstrate that that chiral surface waves can seamlessly propagate forward around various types of defects without experiencing scattering or backward reflection. Remarkably, different types of topological directional couplers are achieved by utilizing the cladding-free Fermi arc surface states in the ideal GMs. We theoretically demonstrate that the physical mechanism of realizing the topological directional couplers is caused by the single coupling channel between the cladding-free Fermi arc surface states and scatterers of the vacuum-GMs system. Moreover, the controllable propagation and topological directional coupling of the cladding-free Fermi arc surface states can be realized by changing the gyromagnetic parameters and boundary configurations in the topological directional couplers. Our work could provide more flexibility for the cladding-free and directional coupling topological devices.