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.

Strong and ultrastrong coupling arise when the exchange of energy between light and matter becomes so large that the two no longer behave independently, instead forming hybrid modes—called polaritons—that combine the characteristics of both. In this work, the researchers entered the ultrastrong coupling regime by tuning surface plasmons—collective electron oscillations—in a semiconductor substrate so that their resonance frequency matches that of the optical lattice vibrations (phonons) of a thin polar crystal layer placed on top. Achieving this alignment produces a flat-band ultrastrong coupling response, meaning that the hybrid polariton states maintain nearly the same energy (frequency) across a broad wavevector range rather than only at specific wavevectors (momenta). This represents a key departure from conventional ultrastrong coupling, where polaritons typically exist only within a narrow region of momentum space.

To demonstrate this effect experimentally, the team used pump–probe nanospectroscopy, a technique in which one optical pulse excites the material and a second, time-delayed optical pulse measures the optical response with nanoscale spatial resolution. A near-infrared pump pulse was used to create mobile electrons in the InAs substrate through photoexcitation. The increased number of mobile carriers shifts the surface plasmon resonance frequency, matching it with the optical phonons of the 50-nm-thick SiC layer deposited above and thereby enabling the flat-band ultrastrong coupling regime. The resulting hybrid surface plasmon–phonon polariton modes were visualized using mid-infrared spectroscopic nanoimaging, which provided information on their frequency–wavevector relationship (dispersion) and confirmed flat-band ultrastrong coupling. The experiments were complemented by theoretical modeling, which confirmed the presence of hybrid polaritons extending over a wide range of momenta.

Reaching strong and ultrastrong coupling over a broad momentum range—effectively generating a large set of hybrid modes simultaneously—could benefit polariton-driven chemistry, where hybrid light–matter states modify chemical energy landscapes, as well as phase transitions induced by strong light–matter coupling, which can reshape a material’s physical properties.