Light Scattering, Absorption, and Refraction Due to High-Order Optical Nonlinearities in Colloidal Gold Nanorods

We investigated the nonlinear (NL) optical behavior of aqueous colloidal dispersions of rod-shaped gold nanoparticles (NPs) with different aspect ratios beyond the usual analysis of third-order nonlinearity. By applying the Z-scan technique, it was possible to observe large NL absorption, which is understood by considering the simultaneous instantaneous and thermo-optical nonlinearity contributions. In addition, light-scattering experiments were performed at high intensities, showing a significant contribution of the fifth-order scattering process that depends on the gold nanorod (Au-NR) aspect ratio. Likewise, the Z-scan measurements reveal that the quintic nonlinearity is also contributing to the refractive behavior, a feature known for spherical gold NPs but not reported for Au-NRs so far. The experimental results illustrate a nonlinearity management procedure based on the strong influence of the aspect ratio of Au-NRs in the excitation regime close to the transverse surface plasmon resonance, even keeping the nanorods' cross-sectional diameter constant. A generalized Maxwell-Garnett model, including the intraband, interband, and hot-electron contributions, was used to clarify the origin of the nonlinearities and corroborate their dependence with the aspect ratio for the first time in the literature.