Ultrafast Dynamics in Strongly‐Coupled Plasmonic Metasurfaces

Strongly coupled plasmonic metasurfaces provide a platform for probing ultrafast light–matter interactions, yet their dynamic response under intense optical excitation remains largely unexplored. Here, we investigate the temporal behavior in metasurfaces composed of adjacent gold dipole and quadrupole resonators on a silicon substrate using time‐resolved mid‐infrared pump–probe spectroscopy. Their near‐field coupling across nanometric gaps facilitates destructive interference and creates a narrow plasmon‐induced transparency (PIT) window. Aside from a transmission increase that lasts several tens of picoseconds as a result of impact ionization, the strongly coupled metasurfaces exhibit an instantaneous sub‐ps transmission spike. We attribute this feature to direct field‐driven interaction that, under strong coupling, simultaneously excites hybrid modes to enable coherent energy exchange—a channel absent in isolated or weakly coupled structures. Notwithstanding these dynamics, the PIT resonance profile evidences a robust, strong near‐field coupling. Numerical analysis reveals the temporal evolution of the bright and dark modes’ damping rates, as well as the inter‐resonator coupling coefficient. Those results establish a well‐grounded mechanism for ultrafast mid‐infrared all‐optical modulation and unveil a previously hidden, ultrafast nonlinearity inherent to strongly coupled plasmonic metasurfaces.