Decoupled-Subspace-Induced Flat Bands in a Photonic Superlattice.

Flat bands in photonic systems enable strong light confinement and enhanced light-matter interactions, yet realizing flat dispersion across an entire Brillouin zone (BZ) remains nontrivial. Here, we propose a practical mechanism for generating BZ-spanning flat bands through the coupling between M propagating modes and N localized resonances. When N>M, the finite-channel model necessarily supports exact flat bands that originate from an effectively decoupled subspace. As a representative example, we implement this mechanism in a plasmonic superlattice, where multiple localized resonances hybridize with surface plasmon polaritons. Angle-resolved reflection measurements directly reveal a quasi-flat band extending across the BZ, in excellent agreement with full-wave simulations. The suppressed group velocity further enlarges the momentum-space separation of quasi-bound-state-in-the-continuum-induced exceptional-point pairs, enabling their experimental observation in a plasmonic platform. Our results establish a practical design strategy for engineering flat bands and non-Hermitian degeneracies, which can be extended to dielectric and hybrid systems across a wide spectral range under appropriate conditions, opening new opportunities for compact photonic devices with enhanced functionality.