  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 Dirac cone sandwiched between two domains of photonic crystals with
opposite topological properties. In this work, we show that a new kind of
large-area topological waveguide states could be created employing the photonic
analogs of 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 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 regarding to the size of the
bandgap 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 robustness of these
large-area waveguide modes against sharp bends are explicitly demonstrated. Our
work enlarges the photonic systems and platforms that could be utilized for
large-area-mode enabled topologically waveguiding.
