Gold films of 20 and 150 nm nominal thickness were characterized by two-beam second-harmonic generation to address their second-order nonlinear optical responses of surface (dipolar) and bulk (higher multipolar) origin. The surface response is enhanced by ~20% in the case of the 20 nm film, as expected due to its higher surface roughness. Surprisingly, the bulk-type response is enhanced to a greater extent, exceeding 80%, and this can be explained by effective quadrupolar nonlinearity arising from the field interaction with the local nonlinearity of nanoscale surface features.
Wang, F. X., Rodriquez, F. J., Albers, W. M., & Kauranen, M. (2010). Enhancement of bulk-type multipolar second-harmonic generation arising from surface morphology of metals. New Journal of Physics, 12(6), . https://doi.org/10.1088/1367-2630/12/6/063009