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

Strong and ultrastrong coupling (SC and USC) are pivotal phenomena in science and technology, where light-matter hybridization opens new avenues for manipulating quantum states, material properties or chemical reactions. Here, we use pump-probe nanospectroscopy for real-space mapping of vibrational USC between optical phonons in a thin SiC layer and surface plasmon polaritons in a semiconductor (InAs) substrate. By adjusting the InAs carrier density through photoexcitation, we align the flat dispersion limit of the SPPs to the SiC TO phonon, yielding hybridized modes in an intriguingly wide wavevector range. This flat-band USC contrasts conventional USC, where hybridization typically occurs in a narrow wavevector range. We further predict flat-band coupling for weak oscillators, demonstrated by SC of molecular vibrations with low-loss surface phonon polaritons at their dispersion limit. Achieving (U)SC over a large wavevector range, and thus many hybrid modes, may benefit polariton chemistry and (U)SC-induced phase transitions.