Topological Photonic Integrated Circuits Based on Valley Kink States

Valley pseudospin, a new degree of freedom in photonic lattices, provides an intriguing way to manipulate photons and enhance the robustness of optical networks. Here, topological waveguiding, refracting, resonating, and routing of valley-polarized photons in integrated circuits are experimentally demonstrated. Specifically, it is shown that at the domain wall between photonic crystals of different topological valley phases, there exists a topologically protected valley kink state that is backscattering-free at sharp bends and terminals. These valley kink states are further harnessed for constructing high-Q topological photonic crystal cavities with tortuously shaped cavity geometries. A novel optical routing scheme at an intersection of multiple valley kink states is also demonstrated, where light splits counterintuitively due to the valley pseudospin of photons. These results can not only lead to robust optical communication and signal processing, but also open the door for fundamental research of topological photonics in areas such as lasing, quantum photon-pair generation, and optomechanics.