Enhancement of the complex third-order nonlinear optical susceptibility in Au nanorods

We experimentally determined the dispersion of the complex third-order nonlinear optical susceptibility χ (3) of Au nanorods over a wide bandwidth (370 - 800 nm). Compared to bulk Au, these nanorods exhibit greatly enhanced nonlinearities that can be manipulated by geometrical parameters. Accurately measuring the χ (3) values of nanostructured metals is challenging because χ (3) is strongly influenced by the local field effects. Hence the current published χ (3) values for Au nanorods have huge variations in both magnitude and sign because Z-scan measurements are used almost exclusively. This work combines pump-probe methods with spectroscopic ellipsometry to show that Au nanorods exhibit strong wavelength dependence and enhanced χ (3) in the vicinity of the longitudinal plasmon mode and explains where the regions of SA and RSA exist and how focusing and defocusing affects χ (3) . In this context, the results highlight the importance of the dispersion of the quantity χ (3) o design plasmonic platforms for nanophotonics applications.