Evidence of High-Order Nonlinearities in Supercontinuum White-Light Generation from a Gold Nanofilm

White-light supercontinuum generation can be readily observed when gold nanostructures are irradiated with short pulses of light. It is believed that the nanostructures enhance the optical fields, which facilitates the supercontinuum white-light generation from the surrounding environment or the substrate. Here, we investigate the different nonlinear processes that contribute to the generation of the supercontinuum from plasmonic nanostructures themselves using a technique that isolates the different nonlinear contributions. By exciting a gold nanofilm with a pair of frequency shifted optical frequency combs, we demonstrate multiple modulation frequencies in the supercontinuum. Their dependence on the excitation intensity reveals that the supercontinuum originates from different orders of nonlinear light-matter interactions. This contrasts with the supercontinuum generation by a cascaded third-order optical nonlinear response in traditional dielectric-based white-light sources. The while-light emission from the gold nanofilm is efficient even under relatively weak excitation indicating that nonlocal effects in nanostructures may facilitate the supercontinuum generation by adding new pathways in the nonlinear interactions. The results provide experimental basis for the understanding of the collective nonlinear response of free-electrons in the metallic nanostructures and associated nonlinear processes, which are crucial in development of nonlinear metasurfaces and nanophotonic devices.