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

摘要

The ultratin (17-25 nm thick) continuous Ni films deposited on K8 glass and ZrO_2 substrates are experimentally studied. The degenerate four-photon spectroscopy showed typical resonances in the dependence of the selfdiffraction efficiency on the wavelength of pump components. It is shown within the framework of the model of lectronic 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 nonlinnear response of Ni films to picosecond laser pulses (20 ps long) in the 620-634 nm spectral range. The interband polarisation relaxation time T_2 was found to be ～200 fs.