Extended terahertz valley-locked surface waves in designer surface plasmon crystals

Abstract Topological valley-locked edge states have been attracting much attention in terahertz (THz) and optical regimes due to their unique unidirectional backscattering-immune feature. However, these one-dimensional edge transports are essentially not compatible to traditional waveguides or devices. In this work, we propose a THz topological waveguide supporting two dimensional valley-locked surface waves based on designer surface plasmon crystals. The waveguide is implemented by designing a sandwich-like A|C|B heterostructure with three domains. The central domain C carrying a Dirac cone in the band structure is topologically trivial. The A and B domains consist of two distinct topological structures with opposite valley-Chern numbers. Unlike topological edge states existing only at the interface of conventional A|B domain wall structure, extended topological valley-locked surface states propagating along the whole B domain are observed in our proposed structure. This heterostructure with designable waveguide width is more flexible for interfacing with existing THz devices, and is quite suitable for high-throughput and high-power-capacity applications. Besides, the unique features of momentum-valley locking and immunity against sharp bends are reserved. This work may promote future topological and traditional integrated functional devices in THz and optical regimes.