Interaction of vibrational modes in ultrathin metallic films following femtosecond excitation

The interaction of vibrational modes in ultrathin metallic films through anharmonicity is investigated. It is shown that under specific relations between vibrational frequencies the enhancement and coupling of certain modes during their excitation are possible. The developed theoretical approach allows us to explain the observed features of transient second harmonic generation signal in ultrathin nickel films under femtosecond pumping. It is found that the dynamics of the process is determined by the vibrational spectrum of confined longitudinal acoustical phonons. The observed thickness dependence of sound velocity in metallic structures containing only a few monolayers, is further interpreted.