Effect of ultrathin Cr layers on surface second-harmonic generation from Cr-SiO₂-Si(001) structures

An ultrathin semitransparent Cr coating layer on oxidized Si(001) significantly alters surface second-harmonic generation (SHG) behavior. The peak locations of the rotational-anisotropy SHG (RA-SHG) signals from Cr coated SiO₂-Si(001) surfaces are shifted by 45° from that of uncoated surfaces consistently for several polarizations, but only for specific photon energies. Moreover, for appropriate polarizations, a peak-to-valley transition of the RA-SHG signal by varying photon energy is observed for Cr coated samples but not for uncoated samples. The time-dependent SHG signal decreases with continuous laser illumination for Cr coated surfaces, but it increases for uncoated surfaces. Through these comparisons, we find that the Cr coating introduces an additional SHG. Interference between the additional and original SHG contributions modifies both the phase and amplitude of the net SHG, and thus the measured spectroscopic and time dependence of SHG. To determine the source of this additional SHG, we perform further surface SHG measurements on a thick Cr film and the SiO₂-Cr interface. We find that it mainly comes from the outermost surface of the Cr coating layer. In addition, we investigate the effect of the thickness of the SiO₂ layer on the SHG response in reflection from Cr-SiO₂-Si(001) structures.