Researchers observe flat-band ultrastrong coupling

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.