Tuning Multi-Polar Plasmonic Modes of NIR Absorbing Long AuNRs Toward Ultra-Sensitive Optical Sensor

Ensembles of large aspect ratio gold rods (AuNRs) are of technological interest due to their extreme optical extinction coefficients at visible and near infrared wavelengths, ultra-fast plasmonic and excitonic character, and self-assembly and field responsivity in concentrated systems that tunes optical and rheological behavior via permanent or temporal order. Furthermore, with increasing aspect ratio and volume, higher order harmonics appear due to phase retardation within the excited particle. Such multipole plasmonic resonances exhibit even larger quality factors due to greater electromagnetic field localization, and are useful in applications pertaining to light signal routers, light manipulators, or multistep enhancers in processes such as second harmonic generation. However, suspensions of large aspect ratio AuNRs often exhibit large polydispersity, which obscures these physicochemical behaviors across the ensemble and at the macroscale. Herein, we discuss synthesis procedures that simultaneously satisfy production scalability with no loss to quality and geometric tunability. Quadrupole and octupole resonances are observed in solutions and films from an ensemble of concentrated AuNRs (>1 mg/mL) with aspect ratios ranging from 5 to 10 and structural dispersity less than 0.1. The relationship between multipole resonance, aspect ratio, particle volume, dielectric environment, and polarization agrees with theoretical predictions.