Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target

Abstract

Structural transformations in a shallow surface region of a bulk Ag (001) target irradiated by a femtosecond laser pulse are investigated in large-scale atomistic simulations and experiments.The simulations reveal a complex interplay of fast laser melting, rapid resolidification, and the dynamic relaxation of laser-induced stresses that leads to the formation of a sub-surface porous region covered by a nanocrystalline surface layer. The generation of the porous region is consistent with the experimental observation of surface "swelling" occurring at laser fluences below the spallation/ablation threshold and may be related to the incubation effect in multi-pulse laser ablation of metals. The nanocrystalline layer is produced by massive nucleation of crystallites triggered by a deep undercooling of the melted surface region experiencing fast quenching at a rate on the order of 10 11 K/s. The predicted surface structure features random crystallographic orientation of nanograins and a high density of stacking faults, twins, and nanoscale twinned structural elements with five-fold symmetry, which suggests high hardness and possible enhancement of catalytic activity of the surface.

Dislocation injection in strontium titanate by femtosecond laser pulses

“…These calculations do assume that the temperature of material where dislocation is gliding remains within 300-900 K. Temperature profile estimates from molecular dynamics simulations in metallic systems, 15 which have much higher thermal conductivities than dielectrics, provide an upper bound on heating due to femtosecond laser ablation. At depths greater than 100 nm below the ablated surface for gold targets, the temperature profile remains below 1000 K. Therefore, the high temperatures experienced during femtosecond laser irradiation are confined to the ejected liquid-like layer in the ablating material and the very near subsurface in metals and dielectric materials, particularly at ablation threshold.…”

“…Femtosecond laser ablation experiments on dielectric materials have shown that material is ejected via a wide range of mechanisms, 11 which can be summarized as thermal and electrostatic in nature. 12 In the process of ejecting material from the surface, compressive and reflected tensile shockwaves are generated that propagate into the material below the ablated surface, as measured and simulated in semiconductors, 13 metals, 14,15 and dielectrics. [16][17][18] In the near-threshold ablation regime, which for STO (Ref.…”

“…Volumetric expansion of the highly excited surface region projects two waves: one compressive and, picoseconds later, a tensile wave that is reflected off the top surface of the expanding surface layer. 15,21 These shockwaves result in glide of dislocations generated near the ablation events. One aim of this paper is to provide a detailed dislocation analysis that bounds the magnitude of the shockwaves necessary to generate and subsequently glide these defects.…”

First-principles assessment of potential ultrafast laser-induced structural transition in Ni

“…Though the solid-to-solid transformation origin is not involved, we also note the theoretical work of Wu et al 21 pointing out the formation of significant HCP domains in a normally FCC structure in Ag after a femtosecond laser irradiation, indicating noble metals can also remains in metastable thermodynamic states after laser exposure. The question of potential ultrafast electronic and structural transformations modifying optical, thermal and mechanical properties is a key issue in a range of observations concerning optical coupling.…”

Solitary Nanostructures Produced by Ultrashort Laser Pulse

“…2. The curves 0.5, 2 have been obtained from the estimates ( 8) and ( 11). In experiments, the bumps appear …”