Collective Plasmon Resonances in Hierarchically Doped Metal Oxide Nanocrystal Metasurfaces

<jats:p>Plasmonic nanocrystal (NC) assemblies produce collective plasmon resonances (CPRs), amplifying light–matter interactions beyond the single-NC limit. Structural and compositional disorder in self-assembled NCs effectively diversify optical responses via inhomogeneous near fields, a heterogeneity that provides a degree of versatility that can benefit applications in sensing and photochemistry. How near-field coupling between spectrally detuned plasmonic NCs sculpts emergent CPR modes and far-field responses remains unexplored. Here, we leverage optical and electron spectroscopies to investigate near-field coupling in hierarchically doped assemblies of plasmonic metal oxide NCs. Like-doped NCs form coherently resonating subsets underlying CPR hybridization, and near-field coupling between dissimilarly doped NCs locally enhances near fields in a way that cannot be explained by NC mixing ratio alone. Optical simulations predict that individual NCs are polarized not at their native localized surface plasmon resonance frequency but instead at each hybrid mode frequency, with coupling either enhancing or suppressing extinction depending on the neighboring NC environment. We experimentally observe this local optical heterogeneity using synchrotron infrared nano-spectroscopy, and visualize the coupling-modified near fields by spatially resolved electron energy-loss spectroscopy. These findings reveal how near-field coupling in heterogeneous NC assemblies governs the emergence of the ensemble response from local to macroscopic scales, providing a conceptual framework for tailored plasmonic metamaterials.</jats:p>