On the resonant SHG response of ultra-thin alkali (K, Rb)-covered Si(1 1 1)-7 × 7

Room temperature adsorption of K either on Si(1 1 1)-7 × 7 or on dielectric SiO2-covered Si(1 1 1) followed by second-harmonic generation (SHG) observation clearly identified a characteristic resonance enhancement typical to Si(1 1 1)-7 × 7. The polarization selected SHG as a function of coverage obtained at different photon energies between 1.17-eV and 1.55-eV show a similar variation of the SH intensity both for K and Rb adsorption on Si(1 1 1)-7 × 7. For submonolayer coverage, wavelength dependent SHG peak observed around 0.4 monolayer (ML) was followed by an SHG peak around 0.9 ML that was less dependent on the wavelength. Previous proposals for an interpretation of the former peak have been questioned. Instead, the former peak has been suggested as being a resonant enhancement due to the transition between alkali atomic-derived electronic states, while the origin of the latter peak remains unidentified. Multiple SHGs of different origins are identified through a phase measurement of SH fields.