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

摘要

Continuous ultrathin Ni films (the thickness is 17-25 nm) on K8 and ZrO{sub}2 substrates have been experimentally investigated. By the degenerate four-photon spectroscopy (DFPS), specific resonant features have been found in the self-diffraction efficiency as a function of the wavelength of the pumping components. The model of a electronic cubic nonlinear susceptibility, which takes into account a real spectrum of Ni electronic states, its quantum-size renormalization and spin splitting, a saturation and selection rules for the electronic transitions, processes of inter-band and intra-band relaxation, has been used to interpret the experimental data obtained. It has been shown that the films' nonlinear response to a picosecond (the pulse duration is 20 ps) laser excitation in the spectral range 620-634 nm is determined by Ni electronic subsystem. The decay time of inter-band polarization T{sub}2～200 fs has been found.