Solitary Nanostructures Produced by Ultrashort Laser Pulse.

Laser-produced surface nanostructures show considerable promise for many applications while fundamental questions concerning the corresponding mechanisms of structuring are still debated. Here, we present a simple physical model describing those mechanisms happened in a thin metal film on dielectric substrate irradiated by a tightly focused ultrashort laser pulse. The main ingredients included into the model are (i) the film-substrate hydrodynamic interaction, melting and separation of the film from substrate with velocity increasing with increase of absorbed fluence; (ii) the capillary forces decelerating expansion of the expanding flying film; and (iii) rapid freezing into a solid state if the rate of solidification is comparable or larger than hydrodynamic velocities. The developed model and performed simulations explain appearance of microbump inside the focal spot on the film surface. The model follows experimental findings about gradual transformation of the bump from small parabolic to a conical shape and to the bump with a jet on its tip with increasing fluence. Disruption of the bump as a result of thinning down the liquid film to a few interatomic distances or due to mechanical break-off of solid film is described together with the jetting and formation of one or many droplets. Developed theory opens door for optimizing laser parameters for intended nanostructuring in applications.

Two-zone elastic-plastic single shock waves in solids.

By decoupling time and length scales in moving window molecular dynamics shock-wave simulations, a new regime of shock-wave propagation is uncovered characterized by a two-zone elastic-plastic shock-wave structure consisting of a leading elastic front followed by a plastic front, both moving with the same average speed and having a fixed net thickness that can extend to microns. The material in the elastic zone is in a metastable state that supports a pressure that can substantially exceed the critical pressure characteristic of the onset of the well-known split-elastic-plastic, two-wave propagation. The two-zone elastic-plastic wave is a general phenomenon observed in simulations of a broad class of crystalline materials and is within the reach of current experimental techniques.