Large-area quantum-spin-Hall waveguide states in a three-layer topological photonic crystal heterostructure

Topological photonic edge states are conventionally formed at the interface between two domains of topologically trivial and nontrivial photonic crystals. Recent works exploiting photonic quantum Hall and quantum-valley-Hall effects have shown that large-area topological waveguide states could be created in a three-layer topological heterostructure that consists of a finite-width domain featuring a Dirac cone sandwiched between two domains of photonic crystals with opposite topological properties. In this Letter, we show that an alternative kind of large-area topological waveguide state could be created employing the photonic analogs of the quantum-spin-Hall effect. Taking the well-used Wu-Hu model in topological photonics as an example, we show that sandwiching a finite-width domain of photonic crystals featuring a double Dirac cone between two domains of expanded and shrunken unit cells could lead to the emergence of large-area topological helical waveguide states distributed uniformly in the middle domain. Importantly, we unveil a power-law scaling related to the size of the band gap within which the large-area helical states reside as a function of the width of the middle domain, which implies that these large-area modes in principle could exist in the middle domain with arbitrary width. Moreover, pseudospin-momentum-locking unidirectional propagations and the robustness of these large-area waveguide modes against sharp bends are explicitly demonstrated. Our work broadens the photonic systems and platforms that could be utilized for large-area-mode-enabled topological waveguiding.