Interaction of vibrational modes in ultrathin metallic films following femtosecond excitation

The interaction of vibrational modes in ultrathin metallic films through an harmonicity is investigated. It is shown that under specific relations betw een vibrational frequencies the enhancement and coupling of certain modes d uring their excitation are possible. The developed theoretical approach all ows us to explain the observed features of transient second harmonic genera tion signal in ultrathin nickel films under femtosecond pumping. It is foun d that the dynamics of the process is determined by the vibrational spectru m of confined longitudinal acoustical phonons. The observed thickness depen dence of sound velocity in metallic structures containing only a few monola yers, is further interpreted. (C) 2001 Elsevier Science Ltd. All rights res erved.