Design of wavelength division multiplexer based on sandwiched topological waveguides in a valley photonic crystal.

Topological photonic systems provide versatile platforms for efficient and robust molding of light. In this work, we present a triangular-kagome hybrid valley lattice, which is employed to construct several sandwiched topological waveguides with unique valley-dependent transmission features, and the underlying physics is revealed. These topologically-protected waveguides have large and tunable working bandwidths, which can be readily engineered just by locally tuning the structure parameters of the single-layer middle region, without the requirement of changing the global lattice as usually done. Finally, as an example for applications, a harpoon-shaped topological wavelength division multiplexer with broad and compact operation bands is realized by taking advantage of these specially-designed topological waveguides. The method and approach presented here are promising in the field of photonic integrated circuits with topological protection, and offer what we believe to be new opportunities for realizing multifunctional photonic devices based on topological waveguide states.