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 frequency detuning of the biharmonic pumping components were observed. A model of the electronic part of the cubic nonlinear susceptibility was constructed. It takes into account the real structure of the spectrum of the electronic states of nickel, its quantum-size renormalisation and spin splitting, interband and intraband relaxation, as well as selection rules for the electronic transitions. For interband polarisation relaxation times T₂ > 300 fs, a qualitative agreement was obtained between the calculated relationships and the experimental data. In the frequency-degenerate case, points at which the nonlinear response should be absent were found on the frequency scale.