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The origin of high‐spatial‐frequency laser‐induced periodic surface structures, known as HSFLs, has always been a controversial topic. HSFLs of sub‐100 nm periodicity and sub‐20 nm amplitude are generated on tungsten by Ti:sapphire femtosecond laser irradiation under four different processing environments (ambient, air at 10 mbar, Ar at 10 mbar, and vacuum at 10−7 mbar). The topography and subtopography analysis together with two‐temperature model–molecular dynamics simulations reveal that HSFLs formation originates from laser‐induced thermal stresses, implying both surface tension and tensile forces are involved. The experimental observation of subsurface cavitation confirms a hydrodynamics‐based origin for these nanostructures. [ABSTRACT FROM AUTHOR]

A self-organization hydrodynamic process has recently been proposed to partially explain the formation of femtosecond laser-induced nanopatterns on Nickel, which have important applications in optics, microbiology, medicine, etc. Exploring laser pattern space is difficult, however, which simultaneously (i) motivates using machine learning (ML) to search for novel patterns and (ii) hinders it, because of the few data available from costly and time-consuming experiments. In this paper, we use ML to predict novel patterns by integrating partial physical knowledge in the form of the Swift-Hohenberg (SH) partial differential equation (PDE). To do so, we propose a framework to learn with few data, in the absence of initial conditions, by benefiting from background knowledge in the form of a PDE solver. We show that in the case of a self-organization process, a feature mapping exists in which initial conditions can safely be ignored and patterns can be described in terms of PDE parameters alone, which drastically simplifies the problem. In order to apply this framework, we develop a second-order pseudospectral solver of the SH equation which offers a good compromise between accuracy and speed. Our method allows us to predict new nanopatterns in good agreement with experimental data. Moreover, we show that pattern features are related, which imposes constraints on novel pattern design, and suggest an efficient procedure of acquiring experimental data iteratively to improve the generalization of the learned model. It also allows us to identify the limitations of the SH equation as a partial model and suggests an improvement to the physical model itself. [ABSTRACT FROM AUTHOR]

Laser processing implies the generation of a material function defined by the shape and the size of the induced structures, being a collective effect of topography, morphology, and structural arrangement. A fundamental dimensional limit in laser processing is set by optical diffraction. Many material functions are yet defined at the micron scale, and laser microprocessing has become a mainstream development trend. Consequently, laser microscale applications have evolved significantly and developed into an industrial grade technology. New opportunities will nevertheless emerge from accessing the nanoscale. Advances in ultrafast laser processing technologies can enable unprecedented resolutions and processed feature sizes, with the prospect to bypass optical and thermal limits. We will review here the mechanisms of laser processing on extreme scales and the optical and material concepts allowing us to confine the energy beyond the optical limits. We will discuss direct focusing approaches, where the use of nonlinear and near-field effects has demonstrated strong capabilities for light confinement. We will argue that the control of material hydrodynamic response is the key to achieve ultimate resolution in laser processing. A specific structuring process couples both optical and material effects, the process of self-organization. We will discuss the newest results in surface and volume self-organization, indicating the dynamic interplay between light and matter evolution. Micron-sized and nanosized features can be combined into novel architectures and arrangements. We equally underline a new dimensional domain in processing accessible now using laser radiation, the sub-100-nm feature size. Potential application fields will be indicated as the structuring sizes approach the effective mean free path of transport phenomena. [ABSTRACT FROM AUTHOR]

Using an electromagnetic approach based on three-dimensional Maxwell's equations coupled with electron density equation, we investigate femtosecond laser interaction with fused silica containing randomly distributed inhomogeneities. By irradiating with linearly, radially, azimuthally, circularly and mixed polarized beams, we visualize and analyze the resulting electron density distributions in glass. The numerical calculations demonstrate the local material excitation to the polarization state of light and underline the nanostructure orientation perpendicular to the local laser polarization. The advantages of using azimuthal- and radial-variant polarizations to produce the nanogratings with desirable characteristics are discussed. Circularly polarized beams are shown to be, on the contrary, beneficial to prevent the nanostructure self-organization. [ABSTRACT FROM AUTHOR]

We investigate initial cumulative irradiation effects leading to variable surface topographies and nanoscale roughness, and triggering eventually the formation of laser-induced periodic surface structures (LIPSS) on Zr-based bulk metallic glasses (Zr41.2Ti13.8Cu12.5Ni10Be25.5 (at%)). We discuss interconnected aspects related to electronic excitation and optical transients, potential variations in the cartography of thermally-driven chemical modifications and topographical features assisting the surface coupling of the electromagnetic field. The transient optical properties of Zr-based BMG surfaces upon ultrafast irradiation, measured by a two-angle time-resolved single-pump doubleprobe ellipsometry method, show a remarkable constancy up to the point of optical damage and rapid gas-phase transition beyond. In intermediate and low exposure conditions, in the vicinity of the damage domain, multi-pulse incubation effects determine the appearance of nanoscale surface structures. The aspects discussed here involve primarily the progression of nanoscale structuring with an increasing number of fs laser pulses starting from a rough surface and evolving towards ordered corrugation. We emphasize the role of initial roughness in determining light coupling and the generation of regular stationary patterns of scattered light, localized energy absorption and spatially-variant ablation or modulated temperature-driven factors for surface relief. From a material perspective, energy dispersive X-ray spectrometry (EDX) analysis shows potential selective vaporization of light elements, leading to gradual compositional changes and proving a spatiallymodulated temperature pattern. A formation scenario is proposed involving interference between the incident laser and scattered light potentially mediated by localized surface plasmons. Finitedifference time-domain (FDTD) simulations are applied to validate the mechanism, showing that LIPSS appear intrinsically related to the surface superposition of electromagnetic waves. [ABSTRACT FROM AUTHOR]

The influence of crystal orientation on the formation of femtosecond laser-induced periodic surface structures (LIPSS) has been investigated on a polycrystalline nickel sample. Electron Backscatter Diffraction characterization has been exploited to provide structural information within the laser spot on irradiated samples to determine the dependence of LIPSS formation and lattice defects (stacking faults, twins, dislocations) upon the crystal orientation. Significant differences are observed at low-to-medium number of laser pulses, outstandingly for (111)-oriented surface which favors lattice defects formation rather than LIPSS formation. [ABSTRACT FROM AUTHOR]

Surface nanostructuring by femtosecond laser is an efficient way to manipulate surface topography, creating advanced functionalities of irradiated materials. Thin-film metallic glasses obtained by physical vapor deposition exhibit microstructures free from grain boundaries, crystallites and dislocations but also characterized by a nanometric surface roughness. These singular properties make them more resilient to other metals to form laser-induced nanopatterns. Here we investigate the morphological response of Zr65Cu35 alloys under ultrafast irradiation with multipulse feedback. We experimentally demonstrate that the initial columnar microstructure affects the surface topography evolution and conditions the required energy dose to reach desired structures in the nanoscale domain. Double pulses femtosecond laser irradiation is also shown to be an efficient strategy to force materials to form uniform nanostructures even when their thermomechanical properties have a poor predisposition to generate them. [ABSTRACT FROM AUTHOR]

The presence of surface oxides on the formation of laser-induced periodic surface structures (LIPSS) is regularly advocated to favor or even trigger the formation of high-spatial-frequency LIPSS (HSFL) during ultrafast laser-induced nano-structuring. This paper reports the effect of the laser texturing environment on the resulting surface oxides and its consequence for HSFLs formation. Nanoripples are produced on tungsten samples using a Ti:sapphire femtosecond laser under atmospheres with varying oxygen contents. Specifically, ambient, 10 mbar pressure of air, nitrogen and argon, and 10−7 mbar vacuum pressure are used. In addition, removal of any native oxide layer is achieved using plasma sputtering prior to laser irradiation. The resulting HSFLs have a sub-100 nm periodicity and sub 20 nm amplitude. The experiments reveal the negligible role of oxygen during the HSFL formation and clarifies the significant role of ambient pressure in the resulting HSFLs period. [ABSTRACT FROM AUTHOR]

A laser-irradiated surface is the paradigm of a self-organizing system, as coherent, aligned, chaotic, and complex patterns emerge at the microscale and even the nanoscale. A spectacular manifestation of dissipative structures consists of different types of randomly and periodically distributed nanostructures that arise from a homogeneous metal surface. The noninstantaneous response of the material reorganizes local surface topography down to tens of nanometers scale modifying long-range surface morphology on the impact scale. Under ultrafast laser irradiation with a regulated energy dose, the formation of nanopeaks, nanobumps, nanohumps and nanocavities patterns with 20–80 nm transverse size unit and up to 100 nm height are reported. We show that the use of crossed-polarized double laser pulse adds an extra dimension to the nanostructuring process as laser energy dose and multi-pulse feedback tune the energy gradient distribution, crossing critical values for surface self-organization regimes. The tiny dimensions of complex patterns are defined by the competition between the evolution of transient liquid structures generated in a cavitation process and the rapid resolidification of the surface region. Strongly influencing the light coupling, we reveal that initial surface roughness and type of roughness both play a crucial role in controlling the transient emergence of nanostructures during laser irradiation. [ABSTRACT FROM AUTHOR]

We report the potential use of non-diffractive Bessel beam for ultrafast laser processing in additive manufacturing environments, its integration into a fast scanning platform, and proof-of-concept side-wall polishing of stainless steel-based additively fabricated parts. We demonstrate two key advantages of the zeroth-order Bessel beam: the significantly long non-diffractive length for large tolerance of sample positioning and the unique self-reconstruction property for un-disrupted beam access, despite the obstruction of metallic powders in the additive manufacturing environment. The integration of Bessel beam scanning platform is constructed by finely adapting the Bessel beam into a Galvano scanner. The beam sustained its good profile within the scan field of 35 × 35 mm 2 . As a proof of concept, the platform showcases its advanced capacity by largely reducing the side-wall surface roughness of an additively as-fabricated workpiece from Ra 10 μ m down to 1 μ m. Therefore, the demonstrated Bessel–Scanner configuration possesses great potential for integrating in a hybrid additive manufacturing apparatus. [ABSTRACT FROM AUTHOR]

A novel additive surface structuring process is devised, which involves localized, intense femtosecond laser irradiation. The irradiation induces a phase explosion of the material being irradiated, and a subsequent ejection of the ablative species that are used as additive building blocks. The ejected species are deposited and accumulated in the vicinity of the ablation site. This redistribution of the material can be repeated and controlled by raster scanning and multiple pulse irradiation. The deposition and accumulation cause the formation of µm-scale three-dimensional structures that surpass the initial surface level. The above-mentioned ablation, deposition, and accumulation all together constitute the proposed additive surface structuring process. In addition, the geometry of the three-dimensional structures can be further modified, if desirable, by a subsequent substractive ablation process. Microstructural analysis reveals a quasi-seamless conjugation between the surface where the structures grow and the structures additively grown by this method, and hence indicates the mechanic robustness of these structures. As a proof of concept, a sub-mm sized re-entrant structure and pillars are fabricated on aluminum substrate by this method. Single units as well as arrayed structures with arbitrary pattern lattice geometry are easily implemented in this additive surface structuring scheme. Engineered surface with desired functionalities can be realized by using this means, i.e., a surface with arrayed pillars being rendered with superhydrophobicity. [ABSTRACT FROM AUTHOR]

Femtosecond laser-induced volume nanograting formation is numerically investigated. The developed model solves nonlinear Maxwell's equations coupled with multiple rate free carrier density equations in the presence of randomly distributed inhomogeneities in fused silica. As a result of the performed calculations, conduction band electron density is shown to form nanoplanes elongated perpendicular to the laser polarization. Two types of nanoplanes are identified. The structures of the first type have a characteristic period of the laser wavelength in glass and are attributed to the interference of the incident and the inhomogeneity-scattered light waves. Field components induced by coherent multiple scattering in directions perpendicular to the laser polarization are shown to be responsible for the formation of the second type of structures with a subwavelength periodicity. In this case, the influence of the inhomogeneity concentration on the period of nanoplanes is shown. The calculation results not only help to identify the physical origin of the self-organized nanogratings, but also explain their period and orientation. [ABSTRACT FROM AUTHOR]

The effect of femtosecond laser shock peening on a model Al-0.3Mn alloy was investigated experimentally and numerically by molecular dynamics. Micro-diffraction experiments performed at synchrotron source revealed the depth profiles of the residual stress and the stored energy of dislocations, a measure of local plasticity. The depth of the maximum compressive stress did not coincide with that of the maximum dislocation energy, which was found at the surface. The interaction between the laser and the metal was simulated with LAMMPS using a two-temperature molecular dynamics package. The model accurately described the equation of state of aluminum and showed nearly equal resolved shear stresses on all slip systems at the wavefront. The dislocation density at a depth of 1 μm, predicted by the Meyers' model [1] , was higher than the experimental data, suggesting possible recovery due to the increased temperature of the sample after repeated shock loading.   [ABSTRACT FROM AUTHOR]

[Display omitted] • Femtosecond laser generated high spatial frequency nanostructures (HSFLs) of sub-20 nm amplitude and sub-100 nm periodicity are generated on tungsten in ambient and vacuum conditions. • Laser generated oxides are differentiated from the native oxides using analytical characterization techniques. • Femtosecond laser assisted oxidation mechanism is established combining two temperature model-molecular dynamics (TTM-MD) simulations and analytical characterization techniques. Despite ultrafast laser-induced topography modification becoming a recognized surface texturing technique in the recent years, comparatively little work has focused on the accompanying chemical alterations. This study aims to fill that gap by investigating the oxidation induced by ultrafast laser irradiation of metals, with a specific focus on tungsten, in different environments: ambient and high vacuum (10-7 mbar). Laser irradiating conditions were chosen to generate so-called High Spatial Frequency Laser Induced Periodic Surface Structures with a sub-100 nm period and sub-20 nm amplitude, as they are supposed to arise in a non-ablative regime. Contact angle measurements, Scanning Transmission electron microscopy cross-sectional images, and x-ray photoelectron spectroscopy analyses were used to investigate the surface chemistry of these structures and reveal significant structural differences between the laser-generated oxides and those accumulated over time from ambient exposure. To establish an oxidation mechanism during laser interaction with tungsten, Two Temperature Model and Molecular Dynamics simulations (TTM-MD) were conducted to determine the temperature evolution over time. The simulation results, complemented by oxygen diffusion data, provide a predictive insight into the development of a thin oxide layer induced by laser irradiation, a conclusion substantiated by the STEM images. These findings suggest that oxidation can occur mostly by solid-state diffusion while the surface is still in the process of cooling down to room temperature following ultrafast photoexcitation. [ABSTRACT FROM AUTHOR]