This Materials World Network (MWN) award supports US-Russian collaborative research to study the formation of nanostructures at surfaces of materials irradiated by a femtosecond laser pulse. The major goal is to provide an atomic-scale understanding of fundamental mechanisms of materials response that can be used to further advance technological applications. The US team at the University of South Florida will develop theory, and perform modeling of the initial stage of absorption of a femtosecond laser pulse, followed by the isochorical compression of the material, propagation of the compression and rarefaction waves, fast non-equilibrium melting of overheated crystal, and slow resolidification of the melt. The theory/modeling will also focus on the fundamental mechanisms and kinetics of bubble nucleation in stretched hot melt, as well as cavitation, ablation, and spallation of the target material. The team at the Russian Academy of Sciences in Moscow and Chernogolovka will perform experimental investigations of response to a femtosecond laser pulse, and formation of nanostructures at the surfaces of several representative classes of materials, and will calculate the optical characteristics of resultant surface nanostructures. A broader impact will be achieved by developing predictive and experimentally validated multi-physics model of laser-induced modification of surface morphology. Reciprocal exchange visits will provide students with a unique opportunity to gain professional experience beyond one nation's borders.
The project focused on fundamental mechanisms of interaction of femtosecond laser pulses (FsLP) with materials surfaces with the goal to develop scientific principles for intelligent design and control of surface nanostructures. The materials response to irradiation by ultra-short laser pulses is complex and involves a sequence of such physical processes as ablation, melting, cavitation, resolidification and spallation. The high-speed flow of irradiated material consists of nano-sized streams, vortices, jets, and bubbles. Such micro-flow of molten material subjected to fast electron cooling can produce a very complicated morphology of the surface layer of the target, including frozen rims, craters, droplets, filaments, shells, and pores/bubbles, which have dimensions comparable with the heating depth – typically ~ 50-200 nm. Such nanostructured surface layers are urgently sought for various optical, electrical, mechanical, chemical, and biological applications. This Materials World Network (MWN) collaborative project involved partners from the University of South Florida (USF) and from two academic institutions of the Russian Academy of Sciences: Landau Institute for Theoretical Physics and Joint Institute for High Temperatures. The collaborative team lead by the USF researchers, investigated the formation of nanostructures at surfaces of metals irradiated by FsLP. The goal was to establish a connection between the parameters of laser radiation, the optical and thermomechanical properties of the target material, and the final structure of the surface layer of the irradiated target through atomic-scale understanding of fundamental mechanisms of laser matter interactions. Experimental trust investigated response to FsLP and formation of nanostructures at the surfaces of several representative classes of materials: simple sp-electron aluminum, transition 3d-metals nickel and tantalum, irradiated within the wide interval of incident fluencies. The accurate measurements of the ablation thresholds and crater depths were combined with extensive characterization of the resultant surface morphologies by electron microscopy and scanning probe techniques. The dynamics of materials response has also been studied by optical methods. The experimental efforts were guided by theory and simulations which studied the initial stage of absorption of pump FsLP within a skin layer, followed by the formation of isochorically compressed material and development of the rarefaction and compression waves under action of FsLP, as well as the fast non-equilibrium bulk melting of overheated crystal, and transition to normal equilibrium melting at the interface between melt and crystal domains. The fundamental mechanisms of propagation of the rarefaction wave, the kinetics of bubble nucleation in stretched hot melt, the cavitation and ablation of the target materials and resolidification resulting in the formation of final surface morphology the investigated at the atomic scale by large-scale molecular dynamics simulations. The collaborative team made several important discoveries: By extending molecular dynamics simulation to the experimental micrometer length scale, the complete dynamics of gold films subjected to ultrashort laser irradiation, culminating in cavitation then ablation of the melt at the front, and crack nucleation then spallation at the rear side of the sample was observed for the first time. The collaborative team simulated at the atomic scale and observed in experiment the formation for complex surface nanostructures upon irradiation of the metallic surfaces by ultrashort laser pulses involving void nucleation in the stretched melt, rapid cavitation and expansion of foam-like melt, breaking of the foam, and subsequent recrystallization and freezing of foam remnants as threads and spikes. The team has discovered nanocavities inside a surface layer of aluminum upon irradiation of the surface by a femtosecond laser pulse. The experimental characterization by femtosecond interference microscopy and transmission electron microscopy was complimented by combined two-temperature hydrodynamics and molecular dynamics simulations which provided a detailed atomic-scale picture of melting, formation of expansion and compression waves, and bubble nucleation in the stretched melt of the aluminum. This research project provided excellent contributions to the education and professional development of graduate and undergraduate students. International collaboration was key for the scientific progress achieved within this program. Interactions and exchanges of information occur on a daily basis and involved extended reciprocal visits. The excellent progress of this collaborative research is reflected in numerous peer-reviewed joint publications and presentations at conferences.
