Material swelling as the first step in the ablation of metals by ultrashort laser pulses

Abstract

The ablation from a single-crystal Al(111) surface with single ultrashort near-infrared laser pulses has been investigated under ultrahigh vacuum conditions. Scanning-electron and atomic-force microscopy of the irradiated surface reveal a surprising development of the material response at increasing fluence: at low fluence, swelling exceeding one hundred nanometers dominates. At higher fluences, a hole is gradually formed in the swollen material, which eventually reaches below the original surface level. The observations indicate the significance of mechanical effects during ablation.Ultrashort-pulse excitation of solid materials has provided significant new information about the dynamics of highly excited matter. For instance, time-resolved x-ray diffraction experiments have shown how transient states of matter allow "ultrafast melting" of semiconductors (for a review, see Ref. 1). For metals, time-resolved electron diffraction experiments of thin aluminum foils have shown atomic disordering on a time scale of a few picoseconds. 2 The improved understanding of light-matter interaction has, ever since the first investigations of femtosecond-laser excitation, been paired with discussions about the possibilities of employing ultrashort-pulse laser excitation for precision manufacturing with minimized heat effects. [3][4][5][6] For further discussions about laser processing, please see Ref. 7.Ultrashort-pulse excitation of bulk metals is normally described using the two-temperature model, 8 which has provided a good explanation of observations in the low-fluence regime of relevance for surface-chemical reactions, 9 and with appropriate modifications also in the fluence regime of relevance for modifications of the native material, e.g., by laser ablation. [10][11][12][13][14][15] The precise response of the metal samples to the high excitation is a topic of ongoing discussion, see Ref. 16 and references therein. In a thermal description, the material is rapidly heated, leaving an overheated liquid, which cools adiabatically and is expelled by evaporation, spallation or-in case the thermodynamic path takes the system close to the critical point-by phase explosion.In this Brief Report, we report the surprising observation that the initial damage of a single-crystal metal surface induced by short-pulse excitation is not associated with significant material removal, rather with a pronounced swelling of the sample. As discussed in detail below, this observation is indicative of the strong mechanical effects that have been suggested to play a role during laser excitation. However, in contrast to predictions by molecular-dynamics simulations of the material response, significant material spallation does not set in immediately, rather a thick layer of material resolidifies in a foamy state with holes and voids, leaving a bump, which extends above the original surface level.Prior to any investigations of the single-crystal Al (111) surface, it is cleaned by a sputter gun with 1 keV Ar + ions for 60 minutes. The Ar atoms are...

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

“…The effect of swelling is not unique to Ag and has also been observed for femtosecond laser irradiation of Al targets [3,62,63], where the presence of sub-surface voids was directly confirmed in transmission electron microscopy (TEM) and scanning electron microscopy (SEM)…”

“…imaging of cross-sections of the swollen areas of the target [3,63]. The smaller thermal conductivity and larger melting depth at fluences close to the spallation threshold in Al, as compared to Ag, reduce the extent of swelling to about a hundred of nanometers and lead to a partial collapse of the voids prior to solidification, as reflected in the flattening of the void shapes parallel to the surface of the target observed in the TEM and SEM images.…”

“…The generation of sub-surface voids is likely to alter the target response to irradiation by subsequent laser pulses and may reduce the threshold fluence for the collective material ejection [3]. To verify this hypothesis, an additional simulation of the 2 nd pulse laser irradiation is performed for the resolidified target obtained by the end of the simulation discussed above.…”

“…The ability of short (pico-and femtosecond) pulse laser irradiation to produce unique surface morphologies [1][2][3][4][5], metastable phases [6][7][8] and microstructure (arrangement of crystal defects) [5,[9][10][11][12][13][14] has been demonstrated in numerous studies and is generally attributed to the conditions of strong electronic, thermal, phase, and mechanical non-equilibrium created in irradiated targets by the fast laser excitation. Detailed understanding of the relations between the basic mechanisms of laser interaction with materials, non-equilibrium processes caused by the fast deposition of laser energy, and the resulting microstructure and properties of laser-treated regions of the targets, however, is still lacking, thus limiting the expansion of laser technologies into the new domain of nanoscale material processing and fabrication.…”

Atomistic Simulation Study of Short Pulse Laser-Induced Generation of Crystal Defects in Metal Targets

“…The explosive decomposition and ejection of a top part of the metal substrate create conditions for rapid cooling and solidification of the remaining target. The rapid solidification, in turn, freezes transient liquid structures generated in the ablation process [24] and results in the formation of complex surface morphology featuring nanospikes and ripples that are similar to the ones commonly produced by laser processing in vacuum, e.g., [33,242,243].…”