Acoustic vibration of metal films and nanoparticles

Acoustic vibration of silver films and nanoparticles in glass, due to impulsive excitation of their fundamental expansion mode, is investigated using a femtosecond pump-probe technique. The vibrational motion is monitored via the induced modulation of the material optical properties which, in the two systems, is shown to reflect that of the real part of the dielectric function. The observed differences in their time responses are attributed to the strong coupling of the nanoparticles with their glass environment. In both systems, the phase and the amplitude of the observed oscillations are in quantitative agreement with an indirect displacive excitation process associated with lattice expansion. Control of the acoustic vibration by two femtosecond pulses is also discussed.