Nonlinear optical properties of Ag nanoparticles embedded in Si

A two-colour sum-frequency generation (2C-SFG) spectroscopy experiment was carried out on an assembly of Ag nanoparticles embedded in a Si3 N4 matrix. The visible beam was fixed either at $\omega _{{\rm vis}}$ = 441.6 nm in the surface plasmon resonance (SPR) of the Ag nanoparticles or at $\omega _{{\rm vis}}$ = 632.8 nm, out of the SPR. The SFG signal was detected while the infrared beam was scanned around 1100 cm-1, a spectral range where an absorption signal has been detected in infrared (IR) reflection absorption spectroscopy. At $\omega _{{\rm vis}}$ = 441.6 nm, the SFG signal put in evidence an electronic resonance contrary to $\omega _{{\rm vis}}$ = 632.8 nm. This second-order nonlinear process is related to the nanostructured system. An interpretation is given in terms of an electronic state arising at the interface of the metallic cluster and the insulating matrix