Unlocking Coherent Control of Ultrafast Plasmonic Interaction

Striking a metallic nanostructure with a short and intense pulse of light excites a complex out‐of‐equilibrium distribution of electrons that rapidly interact and lose their mutual coherent motion. Due to the highly nonlinear dynamics, the photo‐excited nanostructures can generate energetic photons beyond the spectrum of the incident beam, where the shortest pulse duration is traditionally expected to induce the greatest nonlinear emission. Here, these photo‐induced extreme ultrafast dynamics are coherently controlled by spectrally shaping a sub‐10 fs pulse within the timescale of coherent plasmon excitations. Contrary to the common perception, it is shown that stretching the pulse to match its internal phase with the plasmon‐resonance increases the second‐order nonlinear emission by >25%. The enhancement is observed only when shaping extreme‐ultrashort pulses (