Nonlinear response of ultrathin Ni films in degenerate four-photon spectroscopy

The ultrathin (17 — 25 nm thick) continuous Ni films deposited on K8 glass and ZrO₂ substrates are experimentally studied. The degenerate four-photon spectroscopy showed typical resonances in the dependence of the self-diffraction efficiency on the wavelength of pump components. It is shown within the framework of the model of electronic cubic nonlinear susceptibility — taking into account the real structure of the spectrum of electronic states of Ni, its quantum-size renormalisation and spin splitting, the inter- and intraband relaxation, saturation, and the selection rules for the electronic transitions — that the electronic subsystem makes the dominant contribution to the nonlinear response of Ni films to picosecond laser pulses (20 ps long) in the 620 — 634 nm spectral range. The interbandpolarisation relaxation time T₂ was found to be ~200 fs.