Observation of extra-high depolarized light scattering spectra from gold nanorods

We report on depolarized light scattering spectra from suspensions of gold nanorods in terms of the depolarized ratio Ivh/lvv measured from 400 to 900 nm. For separated, highly monodisperse and highly monomorphic samples, we observed spectra with an unprecedented depolarization ratio with a resonance maximum of about 50% at wavelengths of 600-650 nm, below the long-wavelength 780 nm extinction peak. These unusual depolarization ratios are between 1/3 and 3/4 theoretical limits established for small dielectric and plasmon-resonant needles, respectively. Our measurements showed a strong correlation between the resonance depolarization ratio and the ratio of the long-wavelength to short-wavelength extinction resonances. Specifically, for standard seed-mediated-growth nanorods with a round-end-cylinder or a dog-bone shape morphology, the peak-extinction ratios were less than 3 and the resonance depolarization ratio was about 30-35%. The measured extinction and depolarization spectra are in excellent agreement with T-matrix simulations based on TEM data. For silver nanorods, our T-matrix simulations predict multiple-peak depolarization spectra and unique depolarization ratios exceeding the upper dipolar limit (3/4) because of multipole depolarization contributions.