Effect of laser pulse width on material phenomena in ultrathin metal films irradiated by an ultrafast laser: molecular-dynamics study

Using molecular-dynamics computer simulation, we study the material processes occurring for fixed laser fluence in a thin metal film induced by ultrafast laser irradiation. The simulation algorithm combines molecular-dynamics for the atom motion supplemented by an analytical solution of time dependence of the homogeneous electron dynamics. Our results show that the laser pulse duration influences virtually all material processes in the irradiated film: melting, build-up of compressive and tensile pressure, spallation and cluster formation. In particular, for the same laser fluence, a reduction of the laser pulse duration leads to more pronounced pressure peaks in the film, which may induce thermomechanical spallation of the sample, and-at higher fluences, where the ablated material forms a vapour cloud-to an increase in the number of large droplets in the vapour cloud. If the laser pulse duration is smaller than the electron-phonon relaxation time in the material, the latter quantity becomes dominant in limiting the material processes.