Topological States Decorated by Twig Boundary in Plasma Photonic Crystals

The twig edge states in graphene-like structures are viewed as the fourth states complementary to their zigzag, bearded, and armchair counterparts. In this work, a rod-in-plasma system in a honeycomb lattice with twig edge truncation under external magnetic fields and lattice scaling is studied and it is shown that twig edge states can exist in different phases of the system, such as quantum Hall (QH) phase, quantum spin Hall (QSH) phase, and insulating phase. The twig edge states in the negative permittivity background exhibit robust one-way transmission properties immune to backscattering and thus provide a novel avenue for solving the plasma communication blackout problem. Moreover, it is demonstrated that corner and edge states can exist within the shrunken structure by modulating the on-site potential of the twig edges. Especially, helical edge states with the unique feature of pseudospin-momentum locking that can be excited by chiral sources are demonstrated at the twig edges. These results show that the twig edges and interface engineering can bring new opportunities for more flexible manipulation of electromagnetic waves.