Magnon—photon strong coupling at the topological defect in a photonic lattice

Topological photonic structures provide robust and spatially localized modes that are inherently protected against disorder, offering an ideal platform for hybrid light–matter interactions. Here, we experimentally demonstrate strong coupling between collective magnetic excitations (magnons) in a yttrium iron garnet (YIG) sphere and a topological defect mode in a one-dimensional Su–Schrieffer–Heeger (SSH) photonic lattice. By introducing a structural defect into the dimerized photonic lattice, we realize an in-gap state that is both spectrally isolated and spatially confined. When the YIG sphere is positioned at the defect site and the magnon frequency is tuned into resonance with the defect mode, a pronounced normal-mode splitting is observed, confirming the system operates in the strong coupling regime. In contrast, interactions with bulk modes are significantly weaker due to their delocalized field profiles. Our results establish topological defect modes as efficient and controllable interfaces for photon–magnon coupling, paving the way for coherent information processing in topological hybrid magnonic devices.