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⁾ to design plasmonic platforms for nanophotonics applications.