THEORY FOR THE NONLINEAR-OPTICAL RESPONSE OF QUANTUM-WELL STATES IN ULTRATHIN FILMS

We analyze second-harmonic generation (SHG) oscillations due to quantu m-well states in ultrathin overlayer films and their dependence on the electronic and magnetic structure of the film and substrate. Dependin g on the SHG contribution from the surface and the film-substrate inte rface, and the interplay between the wave-function symmetry (via the d ipole matrix elements) and band-structure effects, one obtains differe nt SHG oscillations as a function of film thickness. One may obtain on ly one period, which is twice that observed in the linear magneto-opti cal Kerr effect Lambda(M), but also Lambda(M) and an additional, large r period. Thus we explain, within a unified approach, recent experimen ts on x-Au/Co(0001)/Au(111) and x-Cu/Fe/Cu(001) films, where different characteristic features of the SHG oscillations and different oscilla tion periods were observed.