Optical Sensitivity of Multipole Resonant Gold Nanorods in Solution and Film

Ensembles of large aspect ratio gold rods (AuNRs) are of technological interest due to their extreme optical cross-section at visible and near-infrared wavelengths (C ∼ 10⁴–10⁵ nm²), ultrafast plasmonic and excitonic character (τ ∼ fs–ps), and responsivity to external fields affording ensemble alignment (ns−μs). With increasing aspect ratio and nanorod volume, plasmonic modes beyond dipolar emerge due to phase retardation. Such multipole resonances exhibit larger quality factors due to greater electromagnetic field localization, which is advantageous for optical signal routing, photon manipulation, or multistep processes such as second-harmonic generation, bioimaging, and photocatalysis. However, polydispersity of most as-synthesized AuNRs obscures these physicochemical behaviors across an ensemble, and thus at the macroscale. Herein, we present synthesis procedures for single-crystal AuNRs that simultaneously satisfy production scalability and ultralow dispersity with geometric tunability (aspect ratio:5 to 10; structural dispersity <0.1). Quadrupole and octupole resonances are observed in solutions and nanocomposite films with >10 nM AuNRs. The relationship between multipole resonance, aspect ratio, dielectric environment, temperature, and polarization agrees with theoretical predictions, confirming the foundation for the use of these effects in future technologies.