Topological Edge and Corner States Designed via Meta-Atoms Orientation

Rapid development of topological concepts in photonics unveils exotic phenomena such as unidirectional propagation of electromagnetic waves resilient to backscattering at sharp bends and disorder-immune localization of light at stable frequencies. Recently introduced higher-order topological insulators (HOTIs) bring in additional degrees of control over light confinement and steering. However, designs of photonic HOTIs reported so far are solely exploiting lattice geometries which are hard to reconfigure thus limiting tunability. This article reports a conceptually new mechanism to engineer topological edge and corner states including higher-order topological phases which exploits both electric and magnetic responses of the meta-atoms. Hybridization between these responses gives rise to the difference in the effective coupling which is controlled by the meta-atoms mutual orientations. This feature allows to tailor photonic band topology exclusively via particle alignment and flexibly reconfigure the topological phase. Focusing on the kagome array of split-ring resonators, the topological edge and corner states are experimentally demonstrated in the microwave domain. To highlight the generality of this proposal, the formation of higher-order topological phase is also predicted numerically in a C-6-symmetric lattice of split-ring resonators. These findings provide a new promising route to induce and control higher-order topological phases and states.