  Photonic and acoustic topological insulators exhibiting one-way
transportation that is robust against defects and impurities are typically
realized in coupled arrays of two-dimensional ring resonators. These systems
have produced a series of applications, including optical isolators, delay
lines, and lasers. However, the structures are complicated because an
additional coupler ring between neighboring rings is needed to construct
photonic pseudo-spin. In this work, a photonic analogue of the quantum spin
Hall effect is proposed and experimentally demonstrated in an anomalous Floquet
topological insulator in the microwave regime. This improved design takes
advantage of the efficient and backward coupling of negative-index media. The
results contribute to the understanding of topological structures in
metamaterials and point toward a new direction for constructing useful
topological photonic devices.
