Optical Sensitivity\nof Multipole Resonant Gold Nanorods\nin Solution and Film

Ensembles of large aspect ratio gold rods (AuNRs) are\nof technological\ninterest due to their extreme optical cross-section at visible and\nnear-infrared wavelengths (<i>C</i> ∼ 10<sup>4</sup>–10<sup>5</sup> nm<sup>2</sup>), ultrafast plasmonic and excitonic\ncharacter (τ ∼ fs–ps), and responsivity to external\nfields affording ensemble alignment (ns−μs). With increasing\naspect ratio and nanorod volume, plasmonic modes beyond dipolar emerge\ndue to phase retardation. Such multipole resonances exhibit larger\nquality factors due to greater electromagnetic field localization,\nwhich is advantageous for optical signal routing, photon manipulation,\nor multistep processes such as second-harmonic generation, bioimaging,\nand photocatalysis. However, polydispersity of most as-synthesized\nAuNRs obscures these physicochemical behaviors across an ensemble,\nand thus at the macroscale. Herein, we present synthesis procedures\nfor single-crystal AuNRs that simultaneously satisfy production scalability\nand ultralow dispersity with geometric tunability (aspect ratio:5\nto 10; structural dispersity <0.1). Quadrupole and octupole resonances\nare observed in solutions and nanocomposite films with >10 nM AuNRs.\nThe relationship between multipole resonance, aspect ratio, dielectric\nenvironment, temperature, and polarization agrees with theoretical\npredictions, confirming the foundation for the use of these effects\nin future technologies.