Demonstration of a highly efficient topological vertical coupler

A defect structure is proposed for enhancing the coupling efficiency of vertically incident circularly polarized light in a topological transmission line. In the topological edge-state transmission line based on triangle lattices of hexagons consisting of six nanoholes respecting C6v symmetry in a silicon optical circuit, the vertical coupling rate is improved by removing the nanoholes from one hexagonal cell near the line. The coupling efficiency was evaluated with and without the defect structure. The introduced defect structure operates suitably for focused beams of left- and right-handed circularly polarized light, enhancing the optical communication wavelength bandwidth by up to 10 dB.