SHG enhancement by roughness-induced surface plasmon excitation in alkali-metal overlayers grown on Si(1 1 1)-7 × 7

Thin Alkali metal (AM) films of K, Rb, and Cs were grown on Si(1 1 1)-7 × 7, and second-harmonic generation (SHG) was observed at several wavelengths in the near-IR region at room temperature (RT) in an ultra-high vacuum (UHV) chamber. A large SHG signal was obtained for films deposited at low temperatures. This observation qualitatively agrees with the fact that films deposited at low temperatures are rough. For coverages above the saturation monolayer, resonance enhancement was identified centered at 2.5, 2.4 and <2.2 eV for K, Rb and Cs adsorption, respectively; these observations are in rough agreement with the reported resonance energies of surface plasmons.