Efficient light coupling between conventional silicon photonic waveguides and quantum valley-Hall topological interfaces

Topological interface states gapped between two topological Photonic Crystal (PhC) slabs with different topologies allow backscattering-immune light propagation and quantum valley-Hall (QVH) topological PhC have attracted enormous attention because of its simple geometrical design. In order to integrate the topological interface with practical two-dimensional (2D) optical chips, robust and efficient coupling of light into and out of the QVH topological interface states is crucial. The mismatch in mode profiles of the conventional silicon photonic waveguide and those of topological interface states poses a challenging problem. The coupling efficiency is less than 50% because of the exterior boundaries of a QVH interface, back-reflection and high-order scattering to the end-butt coupled silicon waveguide is inevitable. Here, we present numerical investigations of the light coupling between silicon waveguides and QVH interfaces. By introducing an intermediate PhC line defect, the unwanted scattering channels can be largely suppressed. Mode coupling efficiency is substantially increased without increasing fabrication complexity. The input-to-output total transmission efficiency can reach 95.8% (94.3%) for the bridge (zigzag) QVH interface state coupling with fundamental waveguide modes within near-infrared frequencies. These results can serve as a guide for the future experimental implementation of on-chip 2D topological photonics.