Sum frequency generation (SFG) and difference frequency generation (DFG) are applied to study vibrational resonance of the thiophenol molecule adsorbed on two different gold samples. One sample is made of 17 nm gold nanoparticles (AuNPs) fixed on a silicon substrate that has been previously functionalized with a silane monolayer (aminopropyltriethoxysilane, APTES). This sample is fully characterized through visible reflection spectroscopy and AFM. The second sample is a gold monocrystal also covered with thiophenol molecules. From their comparison, an enhancement factor of 21 is deduced for the SFG signal on AuNPs with respect to the Au(111), related to the surface plasmon resonance (SPR). From a combined analysis of the SFG and DFG spectra, we demonstrate that SFG/DFG spectroscopy is able to identify the nature of the substrate where the molecules are adsorbed. This opens new perspectives for this nonlinear spectroscopy by adding to its well-known intrinsic surface specificity, the ability to selectively probe the chemical layer capping the AuNPs.

We present reflectance-anisotropy spectroscopy (RAS) and surface differential reflectivity (SDR) experiments carried out on well-characterized single-domain nominal silicon (100) surface at room temperature. The results are compared with first-principles calculations which include many-body effects, namely, self-energy and excitonic effects. These strongly modify the line shape of the optical spectra. For a set of the five measurements, the RAS of clean, monohydride, and dyhidride surfaces, and the corresponding SDRs, good agreement between theory and experiment is obtained only when, in the calculation, the electron-hole interaction is accounted for.