Strong magnon-photon coupling enhanced by photonic lattice flat-bands

The high density of states corresponding to photonic flat bands offers a unique avenue for enhancing light-matter interactions, but despite their potential, flat-band continuous modes have largely focused on the weak-coupling or Purcell regimes. Here, we report experimentally achieve strong coupling between the photonic flat-band mode and a magnon mode in a ferrimagnetic spin ensemble. By using one-dimensional Lieb photonic lattices, we reveal that, in the strong-coupling regime, the mechanism underlying flat-band-enhanced interaction is analogous to Dicke superradiance. A localized bright mode is obtained by coherent combination of N degenerate flat-band modes, yielding an enhancement of coupling strength proportional to $\sqrt{N}$ , compared to systems without photonic flat bands. Remarkably, we observe flat-band-induced protection of the strong coupling against lattice-size scaling, an effect we term "coupling pinning". Further enhancement is achieved by sandwiching the spin ensemble between two stacked Lieb layers, resulting in the hybridization of bright modes. Our results establish photonic flat bands as a promising and scalable platform for achieving and sustaining strong light-matter interactions, with potential for large-scale photonic integration and flat-band-enabled functionalities in hybrid systems.