Mechanism of coherent acoustic phonon generation under nonequilibrium conditions

Abstract

We have investigated the dynamics of coherent acoustic phonons generated with ultrafast optical excitation in monoatomic Al films using femtosecond electron diffraction. Associated coherent and thermal lattice motions were measured simultaneously with sub milli-\aa{}ngstr\"om spatial resolution in real time. We show here that the phonon dynamics can be well fitted with a classical harmonic oscillator model using a driving force which includes both electronic and lattice contributions. In particular, the pressure of hot free electrons contributes significantly in driving the coherent acoustic phonons under nonequilibrium conditions when electrons and phonons are not thermalized.