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 inter-resonator coupling. The alternating distance between the nanoslit resonators is controlled with sub-nanometer precision, enabling accurate prediction and experimental observation of topologically protected mid-gap modes via photoemission electron microscopy (PEEM). Our results open the path towards 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.