Optical surface second harmonic measurements of isotropic thin-film metals: Gold, silver, copper, aluminum, and tantalum

Abstract

We have studied optical surface second harmonic generation and have determined the magnitude and relative phase of the second-order susceptibility tensor elements for thermally evaporated gold, and sputtered silver, copper, aluminum, and tantalum. The second harmonic data are understood using an isotropic model of the surfaces. The measurements of the parameters from this nonlinear optical characterization, in conjunction with linear optical characterization of the samples, allow us to extract the elements of the tensors. The typical size of ${\chi }_{\perp }^{\left(2\right)}$ , the tensor element that produces the surface current perpendicular to the surface and is the largest surface element, ranges from $3(\pm 1)\times {10}^{-12}{\mathrm{cm}}^2∕\text{statvolt}$ for an aluminum sample with $24(\pm 4)\mathrm{\AA }\mathrm{rms}$ surface roughness to $1.1(\pm 0.1)\times {10}^{-13}{\mathrm{cm}}^2∕\text{statvolt}$ for a copper sample with $5(\pm 1)\mathrm{\AA }\mathrm{rms}$ surface roughness. Film preparation and associated surface roughness can reproducibly change the values of ${\chi }^{\left(2\right)}$ ; increasing sample roughness increases the magnitudes of the tensor elements. In agreement with previous aluminum measurements, we again find that the tensor elements associated with creating the second harmonic currents normal to the surface are of roughly the theoretically predicted magnitude, but that the elements associated with creating the second harmonic currents parallel to the surface and in the bulk are an order of magnitude smaller than expected.