Recent advances in ultrafast laser technology have enabled the delivery of high pulse energies at MHz repetition rates, but often at peak fluences well above the optimal range for efficient ablation. Temporal pulse shaping can mitigate this issue by distributing energy across multiple sub-pulses separated by defined inter-pulse delays. However, when delays are too short, interaction of preceding pulses with the ablation plume severely reduces ablation efficiency. This study investigates the physical origins of such efficiency losses in double-pulse ablation of copper and steel. Ablation depth and efficiency were measured for inter-pulse delays between 12.2 ns and 231.8 ns. These results were compared with time-resolved pump-probe microscopy measurements of the transient reflectance to quantify plume shielding. A strong correlation is found between second-pulse efficiency and plume transmittance. Plume shielding is identified as the dominant mechanism limiting ablation efficiency in both materials. For copper, redeposition further reduces efficiency, leading to strongly reduced net ablation at inter-pulse delays shorter than 50 ns. Maximum losses of up to 63% in Cu and 21% in steel are observed at interpulse delays of 12.2 ns. Full recovery of second-pulse efficiency is achieved at inter-pulse delays exceeding 150 ns for Cu and 30 ns for steel, corresponding to maximum feasible repetition rates of 6.7 MHz and 33 MHz, respectively. These findings highlight the importance of optimizing inter-pulse delay in double-pulse laser processing and establish time resolved microscopy as a powerful tool for predicting inter-pulse delay dependent ablation efficiency in double-pulse laser processing.

In addition to nanoparticle laser fragmentation in liquids, laser fragmentation of readily available microparticles in liquids has emerged as a promising approach to generate nanoparticles with high efficiency. Despite its advantages, the underlying fragmentation mechanisms, their influence on the nanoparticle size distribution, and the energy efficiency of the process remain poorly understood. In this study, microparticle fragmentation is investigated in single-pulse, single-particle experiments on Au microparticles. Determining the absorbed peak fluence enables an assessment of the process energetics. Pump-probe microscopy in conjunction with nanoparticle size analysis using transmission electron microscopy identifies photomechanical fracture of the molten microparticle volume and photothermal phase explosion of its superheated surface as the fragmentation mechanisms. We find that 83 % of the absorbed laser energy is converted into cavitation bubble energy, while only 2 % is converted into surface energy. The surface energy generated per absorbed laser energy is 20 times higher, and the overall energy efficiency is 30 times higher than for laser ablation of bulk Au in water. This gain originates from the confined microparticle geometry, which enhances photomechanical fragmentation via pressure focusing. These results position microparticle fragmentation in liquids as a fundamentally more energyefficient approach for scalable, laser-based nanoparticle production than laser ablation in liquids.

Laser ablation in liquids enables the synthesis of surfactant-free nanoparticles but remains limited in productivity due to intrinsic constraints imposed by the liquid environment. These constraints include nonlinear optical losses, redeposition, and cavitation-bubble shielding. Temporal intensity shaping of the incident laser energy offers a potential route to mitigate these limitations. Here, GHz bursts of ultrashort pulses are applied to laser ablation of gold in water. By distributing the pulse energy into a sequence of picosecond sub-pulses with an inter-pulse delay of 205 ps, energy can be delivered within the nanosecond time window preceding the onset of cavitation-bubble shielding. With this approach, a specific energy for ablation of approximately 240 J/mm3 is achieved, reaching the physical efficiency limit observed for ultrashort-pulse ablation of Au in air. This efficiency gain is consistent with a mitigation of redeposition and is not accompanied by an increase in cavitation-bubble size or lifetime. Furthermore, GHz-burst irradiation does not broaden the nanoparticle size distribution but instead slightly narrows it. These results demonstrate that GHz-burst LAL enables the physical efficiency limit for ultrashort-pulse laser ablation to be reached in water without increasing cavitation-bubble shielding or compromising the nanoparticle size distribution.

Single-pulse laser ablation thresholds of selected equiatomic Cantor-Wu alloys - FeNi, CoNi, CrFeNi, CrCoNi, and CrMnFeCoNi - are measured for femtosecond and nanosecond pulse durations and interpreted through first-principles calculations of the electronic structure, the electron-phonon coupling, and the electronic thermal conductivity. Alloy synthesis, ablation experiments, and theory are performed consistently on the same set of samples. The absorbed femtosecond thresholds decrease systematically by up to 36

Ultrashort-pulse laser ablation of metals near damage threshold is governed by homogeneous spallation, in which tensile unloading releases a nanometre-thin liquid film whose optical signatures are temporally evolving concentric Newton rings in pump–probe experiments. This well-established picture rests almost exclusively on single-pulse results obtained on ideally flat surfaces, yet application-oriented processing invariably operates in a multi-pulse regime in which each pulse irradiates a surface progressively modified by preceding pulses. Whether homogeneous spallation persists under these conditions has remained an open question. Here we resolve this question using time-resolved pump-probe interferometry applied pulse by pulse to austenitic stainless steel. We show that homogeneous spallation dominates the first pulse, while its contribution is strongly reduced for the second pulse. By the third pulse, Newton rings vanish and sustained surface bulging collapses, with the optical transients fully saturating into a phase-explosion-like signature by the fourth pulse. Fourier-domain coherence analysis rules out roughness-induced decoherence as an optical artefact. Four independent observables, spanning time-resolved and final-state measurements, converge on the same transition after three to four pulses. Spallation-layer formation, widely invoked to explain ultrashort-pulse ablation of metals, is thus a single-pulse phenomenon rather than a multi-pulse ablation mechanism.

Incubation, the systematic reduction of the ablation threshold with pulse number, critically influences ultrashort pulse laser micromachining, yet its microscopic origin remains insufficiently understood despite its widespread relevance in applications. Here, multi-pulse experiments (500 fs pulse duration, 1040 nm wavelength) with fluences ranging from 0.75 to e2 times the ablation threshold and repetition rate of 1 Hz on aluminum and stainless steel were combined with pulse-resolved absorptance from Finite-Difference-Time-Domain simulations to disentangle the roles of global absorption, crater-edge near-field enhancements, and microscopic material weakening. For aluminum, surface roughening leads to an absorption increase reciprocal to the threshold, providing a sufficient explanation of incubation. In stainless steel, however, the threshold decreases despite nearly constant absorption, demonstrating that increased absorption is not a necessary condition for incubation. Edge-localized near-field enhancements provide an early but limited contribution, saturating after a few pulses. A porosity-based description within classical nucleation theory demonstrates that material weakening can only be explained microscopically by defect-induced reductions of the effective penetration depth together with pulse-dependent nucleation rates. These findings establish a microscopic and quantitative framework for incubation, advancing the physical understanding of the transition from single- to multi-pulse ablation, providing the basis for predictive models of multi-pulse ablation with ultrashort-pulses.

Modern ultrafast laser systems often allow for socalled burst-mode operation. This means that trains of ultrashort pulses are emitted with intra-burst repetition rates in the MHz- or GHz-range. These operation modes offer new possibilities in micromachining of different materials such as milling, drilling, or cutting. In this article, we give a short overview of recent developments and cite some achievements in micromachining applications of different materials in these new burst-regimes.

Laser ablation in liquids enables the synthesis of surfactant-free nanoparticles but remains limited in productivity due to intrinsic constraints imposed by the liquid environment. These constraints include nonlinear optical losses, material redeposition, and cavitation bubble-induced shielding. Temporal intensity shaping of the incident laser pulse offers a potential route to mitigate these limitations. Here, ultrashort GHz-burst ablation is applied to laser ablation of gold in water. By distributing the pulse energy into a sequence of picosecond sub-pulses arriving within the nanosecond time window preceding cavitation bubble formation, GHz-burst irradiation enables energy delivery before the onset of bubble-induced shielding. This increases the threshold fluence for nonlinear losses and yields an ablation efficiency enhancement of up to a factor of three compared to single-pulse ablation. Importantly, this efficiency gain is not accompanied by an increase in cavitation bubble size or lifetime. In addition to enhanced efficiency, burst irradiation yields a twofold narrower nanoparticle size distribution. These results demonstrate that GHz-burst ablation is a promising approach to increase productivity while simultaneously improving nanoparticle quality.

Extreme manufacturing with ultrashort-pulse (USP) lasers at the physical limit of precision and efficiency requires understanding ablation dynamics on the picosecond-to-nanosecond timescale. Pump-probe reflectometry (PPR) provides direct access to photomechanical spallation through Newton ring (NR) interference, but this signature vanishes when the spallation layer becomes optically opaque or the ablated material strongly attenuates the probe. Here, we combine PPR with phase-sensitive interferometric pump-probe (PPI) measurements to track the spallation layer in bulk steel, aluminum, and gold. PPI resolves the propagating layer even when the reflected probe signal is suppressed by >95

Predictive modeling of ultrashort-pulse laser ablation requires temperature-dependent material parameters derived from the electronic structure, namely the electronic thermal conductivity, electron–phonon coupling, and heat capacity. These parameters are well documented for elemental metals but remain sparsely documented for alloys, apart from application-relevant exceptions such as stainless steels. The technologically important titanium alloy Ti-6Al-4V is a prominent example, which is still modeled using elemental-titanium values. We compute the electronic transport of hcp Ti and Ti-6Al-4V from first principles, using the Kubo–Greenwood formalism within the Korringa–Kohn–Rostoker coherent-potential-approximation framework, treating chemical and thermal disorder on equal footing. For elemental Ti, the calculated electrical resistivity agrees with independent abinit electron–phonon calculations and experiment, and also reproduces the high-temperature saturation near the Mott–Ioffe–Regel limit. Under electron–phonon nonequilibrium, the electronic thermal conductivity saturates and then decreases with electronic temperature, reaching a maximum of about 2.97 in Ti but only 0.47 in Ti-6Al-4V, a factor of 6.4 lower. In two-temperature-model simulations the alloy and elemental parameter sets yield peak lattice temperatures differing by only about 1.4%, consistent with reported experimental ablation thresholds that differ by about 3%, well within their measurement uncertainties. Replacing the first-principles thermal conductivity with the low-temperature Drude limit shifts the peak lattice temperature by up to 19%, showing that the functional form of the transport model is even more important than the elemental vs alloy distinction for predictive accuracy.

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.

Facing the increasing popularity of 3D-cellular systems as alternative to classical 2D-culture models, Femtosecond Bioprinting (FSB) has been advanced, overcoming former geometrical constraints, now enabling the high precision transfer of individual mammalian cellular spheroids.

Time-resolved microscopy is an established technique for probing the dynamics of laser ablation, thus enabling the exploration of material behavior under extreme conditions produced by laser excitation. Decoding the time-resolved data on the rapid variation of optical properties of a material undergoing nonequilibrium phase decomposition and ejection, however, presents a significant challenge. In this paper, a closely integrated computational and experimental study of laser ablation of FeNi targets is used to establish direct links between the dynamics of laser ablation and the evolution of optical signal in pump-probe experimental measurements. The experiments and large-scale atomistic simulations are performed for a range of fluences covering the onset of material ejection at the threshold for photomechanical spallation and the transition to the phase explosion regime of laser ablation. The connections between the simulations and experiments are established through numerical modeling of the interaction of an electromagnetic wave representing the experimental probe laser pulse with transient states of the ablation plume predicted in the atomistic simulations. The combined modeling and experiments have revealed a complex interplay of processes defining the transient optical properties of the emerging ablation plume, including the oscillations of reflectance due to the interference of parts of the probe pulse reflected from the spalled layer and the newly formed surface of the target in the spallation regime, the disappearance of the interference pattern upon the transition to the regime of phase explosion, nonmonotonous variation of the refractive index of a transient spongy structure of interconnected liquid regions, and the formation of nanoscale hot spots within the expanding spongy structure due to the near-field concentration of electromagnetic field. The results of this study not only provide reliable guidance for the interpretation of optical signals measured in pump-probe experiments but also suggest new ideas for manipulating the ablation plume dynamics to achieve higher efficiency and precision in laser synthesis and processing of materials in the double-or multipulse irradiation regimes.

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.

Curcumin is a natural food additive (nutraceutical), whose bioavailability is impaired by low solubility, a drawback which may be overcome by particle size reduction. Microparticle (MP) laser fragmentation in liquids (LFL) is an emerging production method for sensitive, organic submicron particles (SMPs) and nanoparticles (NPs), as it is characterized by processing at minimal chemical degradation. However, the fragmentation mechanisms need to be understood to tune the MP-LFL process toward a high SMP yield. Therefore, we used pump-probe microscopy (PPM) to elucidate the dynamics of shockwave formation and cavitation bubble growth on single-curcumin particles in correlation with laser fluence utilizing a ps-pulsed laser at 1040 nm. We observed LFL to occur at a threshold fluence of 60 mJ cm(-2). Furthermore, we found a pressure buildup of 308 MPa within the particle, which exceeds the material's tensile strength by one order of magnitude, hinting at strong contributions of photomechanical effects during curcumin MP-LFL. Consecutively, we examined the transfer to MP dispersions where concentration effects during MP-LFL were studied using a ps-pulsed laser at 532 nm and an optimized continuous flat jet (FJ) reactor, which is characterized by the ability to process particularly high concentrations of up to 1000 mg L-1 due to the low liquid layer thickness. An increased mass yield of SMP and NP (determined by UV-vis extinction spectroscopy and SEM) was found at high educt concentrations of 500 mg L-1, which leads to a relative SMP mass yield of 62% and a productivity of 278 mg h(-1), with the potential for quantitative conversion of all MPs into SMPs at optimized illumination conditions (100% yield, 500 mg h(-1)). The outstandingly low by-product fraction (< 0.5%) by far surpasses the standard comminution processes applied today, rendering the MP-LFL technology even more relevant to the food or pharma sector.

Coffee is one of the most popular beverages, characterized by a complex composition and individual flavor profiles, strongly affected by extraction methods. A continuous and scalable cold extraction method based on irradiation of coffee particle dispersions (<200 mu m) with a picosecond-pulsed laser is exploited, analyzing the influence of laser fluence (20-690 mJ cm(-2)) and pulse number (1-10) on the extract composition. The results highlight the generation of a unique volatile aroma profile at low fluence (35 mJ cm(-2)) and a reduced acidity in the liquid phase, which is neither correlated with a global temperature increase nor with changes in the specific surface area. This unique aroma profile is attributed to pulsed laser diffusion enhancement in liquids, resulting in localized heating and substance-specific alteration of the extraction kinetics, with energy consumption three times lower than classical hot coffee extraction and the potential for technical scale processing.

Nutraceuticals provide health benefits and particularly profit from a sensitive, high-purity production process. Microparticle laser fragmentation in liquids is an emerging technique for the contamination-free comminution of organic drugs and nutraceuticals aiming at solubility enhancements. However, current discontinuously operated fragmentation setups suffer from chemical degradation by multipulse laser excitation at high fluence and do not allow for systematic studies of the fragmentation mechanisms. In this work, continuous-flow microparticle laser fragmentation in liquids with ultrashort-pulsed lasers was studied in a circular jet reactor using curcumin and cannabidiol as model substances and single-pulse-per-volume element conditions to compare the fragmentation efficiency for these two nutraceuticals. Fragmentation efficiency based on the yield of submicrometer particles and nanoparticles was quantified using UV-vis extinction spectroscopy, scanning electron microscopy, and analytical centrifugation, while high-performance liquid chromatography determined degradation. We found improved fragmentation efficiency at lower mass concentrations. In all experiments, chemical degradation was minimal (<2%), and increased mass concentration of curcumin enabled ultralow by-product formation of 0.01%. The process selectivity against degradation was defined by the application-relevant descriptor of mole degradation per produced submicrometer particle surface and quantified regarding feedstock mass concentration and nutraceutical type. Cytotoxicity in HepG2 cancer cells was significantly reduced in cells treated with laser-processed curcumin in comparison to unirradiated curcumin controls, and antioxidant effects were proven, ensuring high viability even at high curcumin concentrations.

Nanoparticles (NPs) generated by pulsed‐laser ablation in liquids (PLAL) have benefited many key applications due to their versatility, enlarged surface area, and high purity. However, scaling up NPs production represents one of the main requisites to commercialize this technology. The established upscaling strategy demands high power and repetition rate laser source with fast scanning systems, which are not widely available and costly. Herein, a cost‐effective alternative is proposed, the addition of static diffractive optical elements to achieve parallel processing through the multi‐beam PLAL (MB‐PLAL). In MB‐PLAL, the optimum repetition rate is reduced to compensate laser energy splitting, hence achieving a higher interpulse distance, reducing pulse shielding, and increasing NPs productivity. MB‐PLAL with 11 beams reached a factor 4 productivity increase for iron–nickel alloy (Fe 50 Ni 50 ) NPs compared to the single‐beam setup (0.4–1.6 g h −1 ), and a factor 3 increase for gold (Au) NPs (0.32–0.94 g h −1 ). The scalability of the proposed MB‐PLAL technique setup is confirmed by Au and Fe 50 Ni 50 NPs productivity experiments using 1, 6, and 11 beams, showing a linear increase in productivity.

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.