Highly Localized Plasmonic Jackiw-Rebbi State from a Topological Phase Transition.

Topological photonics provides a route to disorder-robust optical states, among which the Jackiw-Rebbi (JR) state is a prototypical zero-mode bound to an interface. While realized in dielectric platforms, such states typically exhibit extended mode volumes limited by the photonic group velocity. Here we report the experimental observation of a plasmonic JR state in a 1D metal-dielectric grating. By varying a single geometric parameter, we induce a topological band inversion characterized by the change of Zak phases. At the interface between two different topological phases, we observe a sharp midgap state. Its lateral localization length is experimentally bounded above by <5 μm and simulated to be ∼0.7 μm. This enhanced confinement arises from the intrinsically flattened dispersion of plasmonic excitations, which reduces the group velocity and compresses the mode profile while preserving topological protection. Our work establishes a platform that combines topological robustness with compact field confinement.