Selective Ultrafast Excitation and Dephasing of Gap Plasmons into Hot Carriers in Structural Color Generation

Plasmon-induced hot-carriers generated through Landau damping underpin applications in plasmonic optoelectronics, energy conversion, and photocatalysis. However, it remains unclear whether these carriers retain the spatial localization of their parent plasmon modes or rapidly lose this spatial memory through ultrafast scattering and transport. Here, we experimentally probe the spatial and temporal dynamics of plasmon-induced hot-carriers in a strongly confined plasmonic structural color consisting of disordered aluminum nanoparticle ensembles on near-field cavities. Strong interparticle coupling produces hybridized and localized gap plasmon modes with intense nanoscale electromagnetic confinement, which we directly visualize using scattering-type scanning near-field optical microscopy. Ultrafast pump-probe measurements selectively excite a homogeneously broadened sub-ensemble of plasmonic modes, enabling indirect tracking of pump-generated, plasmon-induced hot-carrier dynamics. We observe that hot-carriers undergo only partial spatial delocalization and retain signatures of the initial plasmonic localization for hundreds of picoseconds. The perturbed sub-ensemble reaches maximum line widths of 0.54 ± 0.06 eV (0.36 ± 0.05 eV) under 630 nm (700 nm) excitation. On a phenomenological localization scale from 0 (localized) to 1 (fully delocalized), this corresponds to 0.35 ± 0.10 and 0.09 ± 0.08 for 630 and 700 nm excitation, respectively. The wavelength-dependent fraction shows that the degree of preserved localization is optically tunable, set by the carrier energy. This persistence of spatial memory, likely arising from disorder-induced scattering within the nanoparticle ensemble, reveals an unexpected spatial dimension of hot-carrier dynamics and highlights the role of plasmonic confinement and structural disorder in governing nanoscale energy flow, with implications for hot-carrier-based optoelectronics.