Real-space observation of flat-band ultrastrong coupling between optical phonons and surface plasmon polaritons

The physical and chemical properties of atoms and molecules are governed by their energy levels; tuning these levels can reshape material behavior and drive advances in materials science, chemistry, and photonics. A powerful route is to exploit strong and ultrastrong light–matter coupling (SC/USC), where rapid energy exchange creates hybrid light-matter states. Of particular interest is vibrational strong coupling—light coupled to molecular or lattice vibrations—which has been shown to modify chemical reactivity and is predicted to trigger phase transitions.

 

In this seminar, I will show how pump–probe nanospectroscopy reveals vibrational ultrastrong coupling between transverse optical (TO) phonons in a thin SiC layer and surface plasmon polaritons (SPP) in an InAs substrate. By tuning the SPP dispersion through photoexcitation of carriers in InAs into resonance with the SiC TO phonon, we observe the emergence of a hybrid polariton mode that exhibits hallmarks of ultrastrong coupling (mode splitting > 20%). Remarkably, this hybrid mode spans a broad region of momentum space - unlike conventional SC/USC systems where mode hybridization is confined to a narrow wavevector range near the anticrossing point. This observation introduces the concept of flat-band ultrastrong coupling, where the flat dispersion of the SPP mode enables a large number of TO phonons to participate in the hybridization. Our results provide a pathway toward increasing the density of hybridized states in USC systems, with potential implications for enhancing polariton mediated chemical reactivity and enabling ultrafast, tunable polaritonic devices.