Topologically Protected Photonic Modes in Composite Quantum Hall/Quantum Spin Hall Waveguides*

Photonic topological insulators (PTI) are a new class of structures that can support backscattering-free propagating waves at an interface between topologically non-trivial structures. In the bi-anisotropic meta-waveguide (BMW) PTI designs, by carefully tuning the geometrical and electromagnetic properties, we can emulate the electronic quantum spin Hall (QSH) effect using its electromagnetic analog. A magneto-optical material added to the structure will introduce a quantum Hall (QH) photonic bulk insulator on the BMW basis. The combined photonic systems with multiple topological phases supporting reflection-free edgemodes are proposed and experimentally demonstrated. The spin degree of freedom of such topologically protected edgemodes determines their unique pathways through these systems, free from backscattering and able to travel around sharp corners. As an example of their novel properties, we demonstrate a full 4-port circulator based on composite QH and QSH back-scattering free waveguides.