Wavelength division multiplexing based on the coupling effect of helical edge states in two-dimensional dielectric photonic crystals.

Photonic topological insulators with topologically protected edge states featuring one-way, robustness and backscattering-immunity possess extraordinary abilities to steer and manipulate light. In this work, we construct a topological heterostructure (TH) consisting of a domain of nontrivial pseudospin-type topological photonic crystals (PCs) sandwiched between two domains of trivial PCs based on two-dimensional all-dielectric core-shell PCs in triangle lattice. We consider three THs with different number of layers in the middle nontrivial domain (i.e., one-layer, two-layer, three-layer) and demonstrate that the projected band diagrams of the three THs host interesting topological waveguide states (TWSs) with properties of one-way, large-area, broad-bandwidth and robustness due to coupling effect of the helical edge states associated with the two domain-wall interfaces. Moreover, taking advantage of the tunable bandgap between the TWSs by the layer number of the middle domain due to the coupling effect, a topological Y-splitter with functionality of wavelength division multiplexing is explicitly demonstrated exploiting the unique feature of the dispersion curves of TWSs in the three THs. Our work not only offers a new method to realize pseudospin-polarized large-area TWSs with tunable mode-width, but also could provide new opportunities for practical applications in on-chip multifunctional (i.e., wavelength division multiplexing) photonic devices with topological protection and information processing with pseudospin-dependent transport.