Plasmonic Su-Schrieffer-Heeger chains with strong coupling amplitudes.

Plasmonic many-particle systems with precisely tuned resonances and coupling strengths can exhibit emergent collective properties governed by universal principles. In one-dimensional chains with alternating couplings, known as Su-Schrieffer-Heeger (SSH) systems, this includes the formation of topologically protected mid-gap modes whose intensities localize at the chain's ends. This subwavelength localization at optical frequencies is crucial for achieving strong coupling of mid-gap modes to two-level systems under ambient conditions, extending topological protection to hybrid light-matter states. Here, we have fabricated SSH chains from plasmonic nanoslit resonators with strong interresonator coupling. The alternating distance between the nanoslit resonators is controlled with subnanometer precision, enabling accurate prediction and experimental observation of topologically protected mid-gap modes via photoemission electron microscopy. Our results open the path toward experimental realizations of two-dimensional photonic metasurfaces exhibiting higher-order topological modes that can be strongly coupled to single emitters and quantum materials at ambient conditions.