Far-field optical imaging of topological edge states in zigzag plasmonic chains.

Topological photonics mimicking topological insulators has recently attracted considerable attention. The Su-Schrieffer-Heeger (SSH) model, which is a fundamental topological system, has been experimentally demonstrated in many photonic systems owing to its simplicity. In particular, a zigzag chain, which is described by the SSH model, shows intriguing functionality such as polarization-dependent switching of topological edge states. To date, the far-field imaging of topological edge states in plasmonic chains has not been reported because of the constraint imposed by the diffraction limit. In this study, we experimentally observed the photonic topological edge states of zigzag plasmonic chains composed of metal nanodiscs in the optical region through far-field imaging. Using a chain longer than the diffraction limit, light scattering from the two edges of the zigzag chains was resolved. In the case of such a long chain, it was revealed that tiny gaps of several nanometers between the discs, which are difficult to fabricate, are necessary. Therefore, we propose connected chains and investigate the effect of the shape of the connected part, which reveals that similar topological edge states can be obtained even in the connected chains. The polarization dependence of edge-state imaging showed switching of the systems in trivial and topological phases in the same zigzag chain. Far-field observations serve as an easy and effective tool for the investigation and application of photonic topological edge states.