Optical second-harmonic generation from Ag quantum wells on $\mathrm{Si}\mathrm{}\left(111\right)7\times 7:$ Experiment and theory

Optical second-harmonic generation is shown to be an excellent probe of quantization effects in metallic quantum wells. This technique allows in situ monitoring of samples during growth in ultrahigh vacuum, and in contrast to linear spectroscopy it is not limited by screening processes. As a practical demonstration, the thickness dependence of the second-harmonic signal recorded during growth of Ag on $\mathrm{Si}\mathrm{}\left(111\right)7\times 7$ is presented. The recorded signals show clear oscillations with the film thickness, and the positions of maxima shift towards lower coverage with increasing pump frequency. The essential features of the experimental results are well described by a simple model treating the Ag film as a one-dimensional quantum well. It is demonstrated that excitations from localized quantum well states into states coupled to the Si conduction band continuum are responsible for the main resonances in the nonlinear signal.