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.

No abstract

No 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...

A method for measuring the effective refractive-index differences in a few-mode fiber by applying axial fiber stretching is described. This method represents a straightforward technique for characterization of few-mode fibers. Interference between LP01 and LP11 and in some cases also between LP11 and LP21 are observed in a fiber designed for support of LP01 and LP11. The relative strength of the coupled modes depends on specific splicing characteristics, and in some cases only two modes are seen. The results agree well with theoretical predictions for the fiber under investigation.