  Understanding how light interacts at the nanoscale with metals,
semiconductors, or ordinary dielectrics is pivotal if one is to properly
engineer nano-antennas, filters and, more generally, devices that aim to
harness the effects of new physical phenomena that manifest themselves at the
nanoscale. We presently report experimental results on second and third
harmonic generation from 20nm- and 70nm-thick gold layers, for TE- and
TM-polarized incident light pulses. We highlight and discuss for the first time
the relative roles bound electrons and an intensity dependent free electron
density (hot electrons) play in third harmonic generation. While planar
structures are generally the simplest to fabricate, metal layers that are only
a few nanometers thick and partially transparent are almost never studied. Yet,
transmission offers an additional reference point for comparison, which through
relatively simple experimental measurements affords the opportunity to test the
accuracy of available theoretical models. Our experimental results are
explained well within the context of the microscopic hydrodynamic model that we
employ to simulate second and third harmonic conversion efficiencies, and to
simultaneously and uniquely predict the nonlinear dispersive properties of a
gold nanolayer under pulsed illumination. Using our experimental observations
and our model, based solely on the measured third harmonic power conversion
efficiencies we predict |chi3|~10^(-18)-10^(-17)(m/V)^2, triggered mostly by
hot electrons, without resorting to the implementation of a z-scan set-up.
