Nanospallation induced by a femtosecond laser pulse

Abstract

In the present work phenomena are considered related to the interaction of ultra-short laser pulses, τ L~ 0.1 ps, with metallic targets. The absorption of laser pulse results in formation of thin layer of hot electrons strongly superheated ( i e T T >> ) relative to the ion temperature, i T . Initial thickness of the layer heat d is small, δ heat d , where δ~ 10 nm is the skin layer thickness. Subsequent developments include the following stages: (1) Propagation of electron thermal wave which expands the hot layer heat d ; (2) Cooling of electrons due to energy transfer to cold ions; (3) Onset of hydrodynamic motion that constitutes the rarefaction wave with positive pressure; (4) Further expansion of target material leading to the appearance of negative pressure; and (5) Long separation process which begins with nucleation of voids and goes on to the total separation of spallation plate. The thickness of the plate is ~ 10 nm (we call it nanospallation). Theoretical model involves two-temperature hydrodynamic equations with semiempirical EOS for a metal, electron heat conduction and electron-ion energy exchange. The decay of metastable strongly stretched matter is described by molecular dynamics (MD) simulation with extremely large number of atoms. The experimental setup includes femtosecond chromium-forsterite laser operating in the pump-probe regime. The experiments are performed with gold target. Measured ablation threshold for gold is 1.35 J/cm 2 of incident pump light at inclination 45 0 , ppolarization. Calorimeter measurements give for the absorbed fluence abs F = 0.3 inc F , therefore the threshold value of abs F is 0.4 J/cm 2 .

Ultrashort laser-matter interaction at moderate intensities: two-temperature relaxation, foaming of stretched melt, and freezing of evolving nanostructures

“…The plate exists in the finite range of fluences F abs | abl < F abs < F abs |ev, where F abs |ev is evaporation threshold. 4,[16][17][18][19][20] Let us emphasize that there are a layer of evaporated mass marked by the dashed curve "vapor". Up to ablation threshold, a mass thickness of this layer of pure vapor is small in comparison with depth x abl .…”