Abstract
It is known that under certain conditions the complex surface nano-structures are formed after irradiation on metals by ultrashort optical and X-ray laser pulses. In the paper the mechanism of formation and final geometry of such surface structures are discussed for the case of single pulse acting on a well-polished metal surface. The typical surface structures observed in our experiments and simulations are different from well-known ripples composing a regular pattern generated by excitation of surface plasmons. By contrast with the plasmon mechanism, the observed structures have spacial scales which are order of magnitude less than the used optical laser wavelengths. We demonstrate that such structures are formed after laser irradiation due to the thermomechanical spallation of ultrathin surface layer of melt, rather than the plasmon effects, which are found to be insignificant in given conditions of a single shot and initially smooth surface. Spallation is accompanied by a strong foaming of melt followed by breaking of the foam. After several nanoseconds the foam remnants freeze up with formation of complex nano-structures on a target surface.
Highlights
Response of metal to heating by ultrashort laser pulse was studied using two-temperature (2T) hydrodynamics modeling and molecular dynamics (MD) simulation
Our simulations of Al, Au, Ni and Ta showed that absorbtion of laser energy in range of Fabs ∼ 50 − 200 mJ/cm2 in a skin-layer of 10–20 nm for optical lasers or in an attenuation depth of 20–40 nm for X-ray lasers leads to high electron temperatures, which propagate supersonically into the bulk of metal
Gold was simulated with embedded atom model (EAM) potential developed by a stress-matching method to reproduce material response to a wide range of applied stresses and temperatures [1,2]
Summary
Response of metal to heating by ultrashort laser pulse was studied using two-temperature (2T) hydrodynamics modeling and molecular dynamics (MD) simulation. Small hole appears in membrane when its thickness decreases down to 2-3 interatomic layers as a result of strong expansion and strong stretching of membranes, see figures 2 and 3. In figure 2 this corresponds to appearance of a gap in a membrane 4 in the time range between 768 and 794 ps.
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