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

The ultrathin (17-25 nm thick) continuous Ni films deposited on K8 glass an d ZrO2 substrates are experimentally studied. The degenerate four-photon sp ectroscopy showed typical resonances in the dependence of the self diffract ion efficiency on the wavelength of pump components. It is shown within the framework of the model of electronic cubic nonlinear susceptibility - taki ng into account the real structure of the spectrum of electronic states of Ni, its quantum-size renormalisation and spin splitting, the inter- and int raband relaxation, saturation, and the selection rules for the electronic t ransitions - that the electronic subsystem makes the dominant contribution to the nonlinear response of Ni films to picosecond laser pulses (20 ps lon g) in the 620-634 nm spectral range. The interband-polarisation relaxation time T-2 was found to be similar to 200 fs.