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

Abstract

Ultra-thin continuous Ni films of 5–25 nm thickness on K8 glass and ZrO2 substrates were investigated by two methods of coherent four-photon picosecond (20 ps pulse duration) spectroscopy: the biharmonic pumping technique (BP) and degenerate four-photon spectroscopy (DFPS). Specific resonant features were found in the self-diffraction efficiency as a function of BP components' frequency detuning and of pumping wavelength in DFPS. By the model, taking into account the real electronic structure of Ni, this structure quantum-size renormalization and spin splitting, main inter-band and intra-band relaxation processes, saturation and selection rules for electronic transitions, interpretation of the obtained results was performed. It was shown that the developed model enables one to explain the results obtained if we assume that (i) an electronic subsystem temperature Θc decreases abruptly in a frequency-degenerate case due to efficient direct energy transfer from the electronic subsystem to resonant acoustic phonons and (ii) spin splitting of the sample electronic structure is determined by Θe. The inter-band polarization relaxation time T2 = 200–250 fs was estimated. Copyright © 2000 John Wiley & Sons, Ltd.