High-entropy alloys represent a promising class of materials with potential applications across engineering fields. This study aims to investigate the ultrashort pulse laser ablation dynamics of the high-entropy alloy CrMnFeCoNi in comparison to the conventional austenitic stainless steel AISI 304, focusing on correlating time-resolved pump-probe and post-ablation measurements. Using the transfer-matrix method with pump-probe microscopy, we quantitatively analyze spallation and phase explosion dynamics, linking transient reflectance changes to material-specific ablation mechanisms. Our findings reveal distinct differences in absorption within the ablation plume, with AISI 304 exhibiting a higher absorption of 0.35 compared to 0.2 for CrMnFeCoNi at three times the ablation threshold fluence. This difference indicates a stronger photothermal contribution in stainless steel, accounting for its lower ablation efficiency. Furthermore, the contrast of Newton rings can be well explained by the spallation depth, derived from post-ablation ablation depth measurements. Additionally, we provide insight into the birth (detachment at 10 ps) and death (dissolution between 3 ns and 5 ns) of the spallation layer, enhancing our understanding of transient ablation dynamics. The comprehensive analysis of the transient ablation dynamics, along previously reported post-ablation metrics, provides valuable insights into the laser processing of high-entropy alloys with stainless steel as reference.

Accurate simulations are paramount for deepening our understanding of ultrashort pulse laser ablation, a complex process involving non-equilibrium thermal and material transport on time-scales spanning several orders of magnitude. In response to this need, we propose a novel approach that enhances the use of a readily available finite element method tool for multiphysics simulations by incorporating an equation of state (EOS). This new model, termed the two-temperature solid mechanics model including EOS (SM-EOS), has been meticulously tested against isostatic changes and compared with an experimentally validated two-temperature hydrodynamic simulation (HD). Further comparison was made with classical TTM solid mechanics (SM-ISO) simulations using constant or isobaric material parameters. A mechanism for describing material separation due to spallation is also incorporated in the model. Bulk aluminum serves as prototype within this investigation. Our results show that SM-EOS aligns closely with HD, significantly outperforming the classical SM-ISO simulations. Given its robust performance and ease of implementation, our SM-EOS model is expected to serve as a valuable tool for both research groups and industrial applications, thereby facilitating further investigations into ultrashort pulse laser ablation phenomena. Furthermore, it is expected that our approach could influence other fields in simulating phase transitions and extreme states of matter utilizing solid mechanics calculations.

Owing to their exceptional mechanical, electronic, and phononic transport properties, compositionally complex alloys, including high-entropy alloys, represent an important class of materials. However, the interplay between chemical disorder and electronic correlations, and its influence on electronic structure-derived properties, remains largely unexplored. This is addressed for the archetypal CrMnFeCoNi alloy using resonant and valence band photoemission spectroscopy, electrical resistivity, and optical conductivity measurements, complemented by linear response calculations based on density functional theory. Utilizing dynamical mean-field theory, correlation signatures and damping in the spectra are identified, highlighting the significance of many-body effects, particularly in states distant from the Fermi edge. Electronic transport remains dominated by disorder and potentially short-range order, especially at low temperatures, while visible-spectrum optical conductivity and high-temperature transport are influenced by short quasiparticle lifetimes. These findings improve our understanding of element-specific electronic correlations in compositionally complex alloys and facilitate the development of advanced materials with tailored electronic properties.

This study reports an imaging method for gigahertz surface acoustic waves in transparent layers using infrared subpicosecond laser pulses in the ablation regime and an optical pump-probe technique. The reflectivity modulations due to the photoelastic effect of generated multimodal surface acoustic waves were imaged by an sCMOS camera illuminated by the time-delayed, frequency-doubled probe pulses. Moving the delay time between 6.0nsto11.5ns, image stacks of wave field propagation were created.Two representative samples were investigated: wafers of isotropic fused silica and anisotropic x-cut quartz. Rayleigh (SAW) and longitudinal dominant high-velocity pseudo-surface acoustic wave (HVPSAW) modes could be observed and tracked along a circular grid around the excitation center, allowing the extraction of angular profiles of the propagation velocity. In quartz, the folding of a PSAW was observed. A finite element simulation was developed to predict the measurement results. The simulation and measurement were in good agreement with a relative error of 2% to 5%.These results show the potential for fast and full-field imaging of laser-generated ultrasonic surface wave modes, which can be utilized for the characterization of thin transparent samples such as semiconductor wafers or optical crystals in the gigahertz frequency range.

Surface roughness plays a critical role in ultrashort pulse laser ablation, particularly for industrial applications using burst mode operations, multi-pulse laser processing, and the generation of laser-induced periodic surface structures. Hence, we address the impact of surface roughness on the resulting laser ablation topography predicted by a simulation model and compared to experimental results. We present a comprehensive multi-scale simulation framework that first employs finite-difference-time-domain simulations for calculating the surface fluence distribution on a rough surface measured by an atomic-force-microscope followed by the two-temperature model coupled with hydrodynamic/solid mechanics simulation for the initial material heating. Lastly, a computational fluid dynamics model for material relaxation and fluid flow is developed and employed. Final state results of aluminum and AISI 304 stainless steel simulations demonstrated alignment with established ablation models and crater dimension prediction. Notably, Al exhibited significant optical scattering effects due to initial surface roughness of 15 nm - being 70 times below the laser wavelength, leading to localized, selective ablation processes and substantially altered crater topography compared to idealized conditions. Contrary, AISI 304 with RMS roughness of 2 nm showed no difference. Hence, we highlight the necessity of incorporating realistic, material-specific surface roughness values into large-scale ablation simulations. Furthermore, the induced local fluence variations demonstrated the inadequacy of neglecting lateral heat transport effects in this context.

To understand the dynamics of ultrashort-pulse laser ablation, the interpretation of ultrafast time-resolved optical experiments is of utmost importance. To this end, spatiotemporally resolved pump-probe ellipsometry may be utilized to examine the transiently changing dielectric function of a material, particularly when compared to two-temperature model simulations. In this work, we introduce a consistent description of electronic transport as well dielectric function for bulk aluminum, which enables unambiguous quantitative predictions of transient temperature and density variations close to the surface after laser excitation. Potential contributions of these temperature and density fluctuations to the proposed optical model are investigated. We infer that after the thermal equilibrium of electrons and lattice within a few picoseconds, the real part of the dielectric function mostly follows a density decrease, accompanied by an early mechanical motion due to stress confinement. In contrast, the imaginary part is susceptible to a complicated interaction between time-varying collision frequency, plasma frequency, and a density dependency of the interband transitions. The models proposed in this study permit an outstanding quantitative prediction of the ultrashort-pulse laser ablation’s final state and transient observables. Consequently, it is anticipated that in the future, these models will provide a quantitative understanding of the dynamics and behavior of laser ablation.

In recent decades, laser-matter interactions in ultrafast laser ablation have been extensively studied. Numerous simulations exist today that combine two-temperature modeling with subsequent lattice response, including phase transitions. These simulations primarily provide a qualitative understanding of laser ablation with single pulses, while the precise quantitative prediction of final state and time-resolved observables remains challenging. Moreover, the majority of experimental approaches to study laser ablation are performed with multiple pulses, making it difficult to experimentally validate single-pulse simulations.

Simulation results (raw data), COMSOL simulation files and Python evaluation scripts for the publication entitled "Two-Temperature Solid Mechanics Model incorporating an Equation of State for Readily Accessible Ultrashort Pulse Laser Ablation Simulations". Descripton folders: Example for nomenclature: SM-EOS_005_F075 - SM -> solid mechanics (comsol simulation) - EOS -> simulation with parameters from EOS, respectively ISO -> simulation with iso-parameters - 005 -> pulselength of 0.5 ps - F075 -> Fluence: 0.75 * threshold fluence Structure .txt files: - first 8 rows header - delemiter = ',' - first column is the depth in nm, following columns are the Values of the parameter (each column = 0.1 ps time step) - start time = 2 * pulselength, end time = 100 ps Values: Electron Temperature T_EL.txt in kK Lattice Temperature T_IO.txt in kK Pressure P.txt in GPa displacement.txt in nm Density rho.txt in g/cc phase.txt (1 = solid, 2 = solid+liquid, 3 = liquid, 4 = liquid+gas, 5 = gas, negative sign = metastable) condTrans.txt (1 = solid and 0 = liquid)

The effect of quasi-exponentially decreasing film thicknesses of thin poly-para-xylylene (PPX-N) coatings inside narrow tubes or micro scaled gaps is well known and has been discussed by many authors since the late 1970s. However, for technical applications it is often necessary to provide a longitudinal homogeneous film thickness to ensure the constant properties that are required. In a previous work, it was shown, in principle and for the first time, that a temperature gradient along the tube will effectively counteract the longitudinal decreasing film thickness of the PPX-N coating of the interior wall of a capillary. Therefore, this effect is discussed in theory and the provided model is verified by experiments. Our prediction of a required sticking coefficient curve yields experimentally measured homogeneous film thicknesses and shows a good agreement with the given prognosis. Further, it is shown in theory that there is a maximum achievable homogeneous film thickness in the tube in comparison to a blank surface, which can be understood as a coating efficiency for this type of deposition.

In the context of current state of the art, understanding the laser ablation efficiency decrease for pulse durations exceeding the mechanical relaxation time of a few ps remains a pending research question. A heuristic approach may be used to reveal the role of effective penetration depth on ablation efficiency. Extending familiar contributions of this quantity by a term related to the mechanical surface expansion during pulse irradiation, the relation of ablation efficiency and pulse duration is deciphered. Thus, longer pulses are coupled into an expanded surface, revealing a direct link to the violation of stress confinement. To best demonstrate this hypothesis, a material with high electron-phonon coupling as well as low thermal conductivity, i.e., strong electron locali-zation, is required. These properties are accomplished by high-entropy alloys, and the CrMnFeCoNi alloy serves as prime candidate. We report on single-pulse ablation efficiency experiments of the CrMnFeCoNi alloy which are support by our proposed model.

The interaction of ultrashort laser pulses above the ablation threshold of thin-film indium tin oxide (ITO) is examined with pump-probe microscopy. We are able to observe photomechanical spallation at delay times of hundreds of picoseconds, which plays a stronger role near the ablation threshold of 0.17 J/cm2. A phase explosion may also be observed at tens of picoseconds, playing a stronger role for increasing peak fluences. As one exceeds the material removal efficiency maximum near 0.6 J/cm2, a second spallation is observable in the center of the irradiated spot at a delay time of one nanosecond and corresponds to a crater depth of 50 nanometers. No discernable ridge formation has been observed. We recommend an industrial processing window of at least two pulses per position with a peak fluence between 0.6-1.0 J/cm2.

In recent years, high entropy alloy research has experienced increased interest and it was found that some of these materials have extraordinary properties. High entropy alloys also show an increased damage resistance to high-energy particle irradiation, mainly due to effects caused by the increased configuration entropy. So far, no detailed studies have been carried out regarding the interaction with high-energy electromagnetic radiation, particularly by means of lasers. In this work, we compare results of ultrashort-pulse laser-matter interaction of the CrMnFeCoNi alloy (Cantor alloy), the most researched representative of this material group, with the conventional alloy stainless steel AISI 304. Since metals can in general be processed efficiently with ultrashort pulses, which is of particular interest for industrial applications, we performed our experiments with single infrared sub-picosecond pulses. The crater surface morphology and process energetics are discussed in detail and the validity of established ablation models is investigated. We find that the damage threshold of the CrMnFeCoNi alloy is slightly lower than that of AISI 304 and consequently CrMnFeCoNi alloy shows an increased ablation volume. Therefore, the high entropy alloy CrMnFeCoNi can be processed efficiently with ultrashort-pulse lasers.

Depositing a film via chemical vapor deposition results in superior conformity compared with other deposition techniques, primarily due to the unique chemical interactions between the surface and the reactive compounds. This technique requires a readily accessible surface and so, if the transport of the reactive species is impeded, irrespective of whether this depletion is caused by diffusion or convective flow, a homogeneous layer thickness cannot be achieved. This is often the case when applying films to the interiors of tubes, especially tubes with a dead-end, such that the inevitable loss of film-building components leads to a drop in thickness along the deposition length. The present work examined the deposition of the organic polymer poly-p-xylylene, using a reactor with dimensions that were large compared with the mean free path and tubes in which this factor (the Knudsen number) becomes unity, such that the deposition can be approximately described with the continuum model.A so-called temperature seesaw was employed to mitigate variations in layer thickness by generating an opposing temperature gradient. It was found that, under a vacuum of several tens of mTorr, the polymer could be deposited on the interior wall of a tube with an aspect ratio of at least 100 with an accuracy of ±7.5 %. The true ceiling temperature for the N derivate of this polymer was also determined to be 70±2 °C.