Surface and bulk contributions to the second-order nonlinear optical response of a gold film

We use two-beam second-harmonic generation to separate the surface (electric dipole origin) and bulk (magnetic dipole and electric quadrupole origin) contributions to the second-order nonlinear optical response of an isotropic gold film. The bulk response is unambiguously observed and explained by momentum damping of electrons in a free-electron model. Although bulk effects could be enhanced by inhomogeneous local fields in metal nanostructures and have been used to model second-harmonic generation from metamaterials [Y. Zeng et al., Phys. Rev. B 79, 235109 (2009)], we find that surface effects dominate the nonlinearity. Our quantitative results for the surface and bulk parameters set the baseline for future descriptions of the second-order response of nanostructured metals.