Figure 8.

SFG/DFG signals as a function of the UV-Visible optical/electronic properties of nanostructured gold. (a) Comparison between thiophenol/Au(111) interface and thiophenol/AuNPs/APTES/Si interface (Visible = 532 nm, IR = 3 µm) in the C–H spectral range (stretching vibration mode of the phenyl ring). In the AuNP case, SFG intensity is enhanced by a factor F = 21, corresponding to evidence of LSPR-SFG coupling. The DFG spectrum is also displayed to illustrate the different interference role of the $s-d$ interband electronic transition of gold on the SFG/DFG spectral shapes. (b) SFG/DFG spectra of thiophenol/AuNPs/APTES/Si interface (Visible = 532 nm, IR = 10 µm) in the spectral range of C–C vibration modes of the phenyl ring, showing a high SFG and DFG response of this strongly Raman-active modes, also shape-modulated by the interband electronic $s-d$ transition. (c) UV-Visible spectrum of the thiophenol/AuNPs/APTES/Si interface. The presence of silicon induces a shape-reversal of the optical response, dominated by the silicon reflectivity. The LSPR maximum is located at 525 nm and couples more or less with the SFG/DFG energy (in addition to the incident visible beam one) as a function of the probed IR spectral range. The LSPR-SFG coupling is favored in the (b) case. Figure panel is adapted from references [64,65].