Extension of Discrete-Dipole Approximation Model to Compute Nonlinear Absorption in Gold Nanostructures

Discrete-dipole approximation (DDA) model has been widely used to provide quantitative predictions on the linear optical absorption of metallic nanostructures (MNS) irrespective of their geometry. Here, we demonstrate that it can be extended to the computations of MNS' nonlinear light absorption. In the extended DDA (e-DDA) modeling, Drude's dielectric function (or standard dielectric function) for given MNS in the dark is employed for the computation of linear absorption in an ensemble of the MNS. As excited by intense laser light, the dielectric function is altered in the presence of photoexcited electrons in the MNS. With the altered dielectric function and quasi-equilibrium approximation, the DDA model is reapplied to acquire the quantitative simulation on the nonlinear optical absorption. The results of e-DDA modeling are in good agreement with experimental data for gold nanorods and nanospheres.