Coherent four-photon picosecond spectroscopy of ultra-thin nickel films

Ultra-thin continuous Ni films of 5-25 mn thickness on K8 glass and ZrO2 su bstrates were investigated by two methods of coherent four-photon picosecon d (20 ps pulse duration) spectroscopy: the biharmonic pumping technique (BP ) and degenerate four-photon spectroscopy (DFPS), Specific resonant feature s were found in the self-diffraction efficiency as a function of BP compone nts' frequency detuning and of pumping wavelength in DFPS, By the model, ta king into account the real electronic structure of Ni, this structure quant um-size renormalization and spin splitting, main inter-band and intra-band relaxation processes, saturation and selection rules for electronic transit ions, interpretation of the obtained results was performed. It was shown th at the developed model enables one to explain the results obtained if we as sume that (i) an electronic subsystem temperature Theta(c) decreases abrupt ly in a frequency-degenerate case due to efficient direct energy transfer f rom the electronic subsystem to resonant acoustic phonons and (ii) spin spl itting of the sample electronic structure is determined by Theta(e). The in ter-band polarization relaxation time T-2 = 200-250 fs was estimated. Copyr ight (C) 2000 John Wiley & Sons, Ltd.