Researchers from CIC nanoGUNE, in collaboration with the Donostia International Physics Center (DIPC) and the Center for Materials Physics (CFM), have experimentally observed and theoretically verified flat-band ultrastrong coupling between optical phonons and surface plasmon polaritons. Published in Nature Materials, the study reveals a previously unexplored regime of light–matter interaction with potential applications in polariton-driven chemistry, materials science, nanophotonics, and quantum engineering.