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Industrial use of ultrashort pulse surface structuring would significantly increase by effective utilization of the average laser powers available currently. However, the unexplained degradation of surfaces processed with numerous pulses at high average laser power makes this difficult. Based on a systematic experimental study, the structure formation underlying such surface degradation is investigated. Furthermore, a hierarchical structural formation model that bridges the gap between laser‐induced periodic surface structures and surface degradation is presented. Contrary to expectations based on previous research, less structure formation on titanium was observed for higher laser fluences. As a possible reason, enhanced electron diffusion with increasing intensity is investigated within the framework of the two‐temperature model. Our findings provide a deeper understanding of the microscopic mechanisms involved in surface structuring with ultrashort pulses. The authors explain the microscopic origin of bumpy surfaces limiting the ablation rates in ultrashort pulse laser micromachining. Experimental observations supported by theoretical modeling reveal the origin of these bumps to be in an inhomogeneous energy deposition inducing periodic surface structures. They gradually rise with increasing pulse number. Convective growth of surface grooves leads to the final structures.

Laser processing with ultra-short double pulses has gained attraction since the beginning of the 2000s. In the last decade, pulse bursts consisting of multiple pulses with a delay of several 10 ns and less found their way into the area of micromachining of metals, opening up completely new process regimes and allowing an increase in the structuring rates and surface quality of machined samples. Several physical effects such as shielding or re-deposition of material have led to a new understanding of the related machining strategies and processing regimes. Results of both experimental and numerical investigations are placed into context for different time scales during laser processing. This review is dedicated to the fundamental physical phenomena taking place during burst processing and their respective effects on machining results of metals in the ultra-short pulse regime for delays ranging from several 100 fs to several microseconds. Furthermore, technical applications based on these effects are reviewed.

Laser-based directed energy deposition using metal powder (DED-LB/M) offers great potential for a flexible production mainly defined by software. To exploit this potential, knowledge of the process parameters required to achieve a specific track geometry is essential. Existing analytical, numerical, and machine-learning approaches, however, are not yet able to predict the process parameters in a satisfactory way. A trial-&-error approach is therefore usually applied to find the best process parameters. This paper presents a novel user-centric decision-making workflow, in which several combinations of process parameters that are most likely to yield the desired track geometry are proposed to the user. For this purpose, a Gaussian Process Regression (GPR) model, which has the advantage of including uncertainty quantification (UQ), was trained with experimental data to predict the geometry of single DED tracks based on the process parameters. The inherent UQ of the GPR together with the expert knowledge of the user can subsequently be leveraged for the inverse question of finding the best sets of process parameters by minimizing the expected squared deviation between target and actual track geometry. The GPR was trained and validated with a total of 379 cross sections of single tracks and the benefit of the workflow is demonstrated by two exemplary use cases.

Nowadays it is a big challenge to exploit the available average power of ultrashort-pulsed laser systems for the production of high quality structures in metals. A dual process strategy of alternating GHz bursts and conventional processing helps to avoid adverse effects that occur at higher average power. The combination of polishing and ablation leads to higher ablation rates. This new strategy also meets higher quality requirements in terms of surface and edge quality. Due to the spatially and temporally limited re-melting with ultrashort pulses, no strong melt formation occurs during polishing. Consequently, this approach enables a high increase of the ablation rate without lowering the precision of ultrashort pulse milling. Keywords: burst ablation, surface structuring, pulsed polishing, surface quality, milling, dual process strategy, GHz bursts, bumps, edge deepening

In ultra-short pulsed laser ablation of metals, a considerable amount of the incident laser energy remains in the workpiece as residual heat. When using high repetition rates, this can cause severe damage due to heat accumulation effects. In this paper, the amount and the spatial distribution of the residual heat deposited by a spatially Gaussian-shaped laser pulse are investigated by means of calorimetric measurements

The influence of the burst mode is investigated for steel, copper, silver and gold. It is shown that the gain in the removal rate, reported in literature, is generally caused by the lower fluence of the single pulses in the burst which is nearer the optimum value showing higher efficiency. Therefore, identical or even better results are obtained with single pulses of the same fluence but at higher repetition rate. However, for copper, silver and gold and a three pulse burst the second pulse hinders the ablation of the first pulse and it is supposed that it even re-deposits material. But compared to the first pulse the energy of the third pulse is much better converted to removed material leading to a real gain of 16% in efficiency for copper. An explanation of this typical behavior and its absence in the case of steel can be given by models of the ablation process dealing with a bulging and/or spalled layer and plasma as well.

A model for predicting the borehole geometry for laser drilling is presented based on the calculation of a surface of constant absorbed fluence. It is applicable to helical drilling of through-holes with ultrashort laser pulses. The threshold fluence describing the borehole surface is fitted for best agreement with experimental data in the form of cross-sections of through-holes of different shapes and sizes in stainless steel samples. The fitted value is similar to ablation threshold fluence values reported for laser ablation models.

Laser drilling is a processing technology applicable for creating holes in various materials. By combining an ultrashort pulsed laser with a helical drilling optics, it is possible to produce high-quality holes with sharp, burr-free edges in a large variety of borehole geometries. To master this highly flexible process which is determined by a multitude of variables, a mathematical description of the problem is advantageous. A simulation model is presented for calculating the final borehole geometry for a given set of laser, process, and material parameters. The model was validated by comparison to boreholes drilled into four different materials: stainless steel, copper, silicon, and aluminum oxide ceramic. By inserting literature values for the refractive index and the ablation threshold fluence, a good agreement between model and experiment can be achieved for holes of different shape and size. Keywords: ablation threshold, helical drilling, laser materials processing, simulation, ultrafast lasers

This study systematically investigated the sensitivity of the phobic attention system by measuring event-related potentials (ERPs) in spider-phobic and non-phobic volunteers in a context where spider and neutral pictures were presented (phobic threat condition) and in contexts where no phobic but unpleasant and neutral or only neutral pictures were displayed (phobia-irrelevant conditions). In a between-group study, participants were assigned to phobia-irrelevant conditions either before or after the exposure to spider pictures (pre-exposure vs post-exposure participants). Additionally, each picture was preceded by a fixation cross presented in one of three different colors that were informative about the category of an upcoming picture. In the phobic threat condition, spider-phobic participants showed a larger P1 than controls for all pictures and signal cues. Moreover, individuals with spider phobia who were sensitized by the exposure to phobic stimuli (i.e. post-exposure participants) responded with an increased P1 also in phobia-irrelevant conditions. In contrast, no group differences between spider-phobic and non-phobic individuals were observed in the P1-amplitudes during viewing of phobia-irrelevant stimuli in the pre-exposure group. In addition, cues signaling neutral pictures elicited decreased stimulus-preceding negativity (SPN) compared with cues signaling emotional pictures. Moreover, emotional pictures and cues signaling emotional pictures evoked larger early posterior negativity (EPN) and late positive potential (LPP) than neutral stimuli. Spider phobics showed greater selective attention effects than controls for phobia-relevant pictures (increased EPN and LPP) and cues (increased LPP and SPN). Increased sensitization of the attention system observed in spider-phobic individuals might facilitate fear conditioning and promote generalization of fear playing an important role in the maintenance of anxiety disorders.

Learning to avoid threats often occurs by observing others. Most previous research on observational fear learning (OFL) in humans has used pre-recorded standardized video of an actor and thus lacked ecological validity. Here, we aimed to enhance ecological validity of the OFL by engaging participants in a real-time observational procedure (35 pairs of healthy male friends, age 18–27). One of the participants watched the other undergo a differential fear conditioning task, in which a conditioned stimulus (CS+) was paired with an aversive electric shock and another stimulus (CS−) was always safe. Subsequently, the CS+ and CS− were presented to the observer to test the OFL. While the friend’s reactions to the shock elicited strong skin conductance responses (SCR) in all observers, subsequent differential SCRs (CS+ > CS−) were found only when declarative knowledge of the CS+/US contingency (rated by the participants) was acquired. Contingency-aware observers also showed elevated fear potentiated startle responses during both CS+ and CS− compared to baseline. We conclude that our real-time procedure can be effectively used to study OFL. The procedure allowed for dissecting two components of the OFL: an automatic emotional reaction to the response of the demonstrator and learning about stimulus contingency.