Four-photon spectroscopy of ultrathin nickel films involving picosecond biharmonic pumping

Ultrathin (5-25 nm thick) nickel films were investigated experimentally by the picosecond biharmonic pumping method. Characteristic side resonances in the wing of the dependence of the self-diffraction efficiency on the frequ ency detuning of the biharmonic pumping components were observed. A model o f the electronic part of the cubic nonlinear susceptibility was constructed . It takes into account the real structure of the spectrum of the electroni c states of nickel, its quantum-size renormalisation and spin splitting, in terband and intraband relaxation, as well as selection rules for the electr onic transitions. For interband polarisation relaxation times T-2 > 300 fs, a qualitative agreement was obtained between the calculated relationships and the experimental data. In the frequency-degenerate case, points at whic h the nonlinear response should be absent were found on the frequency scale .