Maximilian Spellauge

Ultrashort pulsed laser ablation in liquids allows the synthesis of surfactant-free nanoparticles. However, redeposition of ablated material and nonlinear attenuation in the liquid reduce process efficiency on sub-nanosecond timescales. On longer timescales, shielding by transient and persistent microbubbles can reduce process efficiency. Here, we investigate the ablation of FeNi in water by spatio-temporal distributing the laser intensity. We apply double-pulses with an inter-pulse delay of 600 ps and apply scanning to avoid bubble shielding and employ incubation by irradiating the same sample position multiple times. This isolates the contributions of redeposition and nonlinear losses. In total, we reach a power specific productivity of 75 mg/(W×h) comparable to a specific energy for ablation of about 400 J/mm³ with a pulse duration of 10 ps and a total peak fluence of 10 J/cm². This is a significant improvement to the single-pulse value of 670 J/mm³ and the highest reported value of 2000 J/mm³. The results do not indicate suppression of redeposition by the second pulse at 600 ps inter-pulse delay. However, employing double-pulses produces 30 % smaller nanoparticles. These results demonstrate that spatio-temporally distributing intensity is an effective way to increase the productivity of laser ablation in liquids towards the physical limit.

Laser-based selective thin film removal is a key process in microelectronics manufacturing, where functional layers must be removed with high precision while preserving the integrity of the underlying substrate. In many technologically relevant systems, thin films are covered by additional capping layers, whose influence on the removal mechanisms remains poorly understood. In this work, we investigate the selective laser removal of VOx and AZO-capped VOx thin films deposited on silicon using time-resolved pump–probe microscopy in combination with final state crater analysis. For both systems, thin film removal is governed by indirectly induced ablation, whereby laser energy is predominantly absorbed in the silicon substrate, leading to stress-driven film lift-off. At higher fluences, the onset of phase explosion induces substrate damage and prevents selective thin film removal. The presence of the AZO capping layer reduces the processing fluence window for selective thin film removal by approximately 30%, enhances the confinement of the molten substrate and suppresses silicon substrate removal. In addition, residual fragments of the capping layer remain on the surface after processing. These findings demonstrate that capping layers significantly alter both the process window and the underlying removal dynamics and must therefore be carefully considered in the design and optimization of laser-based thin film removal processes.

Laser ablation in liquid is a versatile and environmentally friendly method for producing nanoparticles from a wide range of materials. While recent advances have significantly increased productivity, laser ablation in liquid remains less energy‐efficient than ablation in air. Simulations have suggested that this discrepancy arises from the redeposition of ablated material, but experimental validation has so far been lacking. Here, we quantify the energy efficiency of laser ablation in liquid and provide direct experimental verification of redeposition. An absorption‐corrected one‐to‐one comparison of single‐pulse ablation in air and water shows that the absorption‐corrected specific energy for ablation increases fourfold in water due to redeposition, from 50 J mm ⁻³ to 200 J mm ⁻³ . For multi‐pulse conditions, the required energy for ablation based on the incident pulse energy amounts to 431 J mm ⁻³ , corresponding to an order‐of‐magnitude improvement compared to the best ultrafast LAL value of 3333 J mm ⁻³ reported to date. These findings establish the practical potential of LAL for green and energy‐efficient nanoparticle synthesis.

The targeted generation of laser‐induced periodic surface structures (LIPSS) is a powerful technique in modern surface engineering. While the dominant role of optical scattering and surface electromagnetic ways in LIPSS formation is widely acknowledged, formation mechanisms still pose open questions, particularly regarding material‐specific differences in uniformity and prominence. Here, a comparative pulse‐to‐pulse analysis of aluminum and stainless steel is presented, chosen to represent relevant metals that develop rough and smooth crater nanomorphologies, respectively, following single‐pulse irradiation. The findings reveal that the initial nanomorphology governs subsequent LIPSS development. After the initial pulse, aluminum forms pronounced spiky nanostructures characterized by elevated local surface gradients within the ablation crater, leading to strong scattering and localization of surface electromagnetic waves. This suppresses periodic intensity distribution and prevents the formation of LIPSS. In contrast, stainless steel retains a smooth crater morphology after the first pulse, enabling well‐defined systematic progression from high‐frequency to low‐frequency LIPSS. Supported by finite‐difference time‐domain simulations on measured surface topographies, the results quantitatively link initial nanomorphology to optical interactions, fundamentally defining material‐dependent pathways for LIPSS formation. Beyond previous explanations relying on macroscopic material parameters, this nanomorphological perspective provides an additional, yet direct explanation for inherent differences in LIPSS susceptibility among various metals.

Laser synthesis and processing of colloids (LSPC) in liquids has gained widespread applications in producing nanomaterials of different classes of solids. While the technical processes in different cases of ablation, fragmentation or colloidal fusion may look macroscopically different in each application, the underlying fundamental mechanisms are always the same cascade of laser interaction with matter, non-thermal or thermal energy deposition, phase transitions, and the subsequent structure formation processes. Disentangling these mechanisms represents a veritable challenge, as ultrafast and structurally sensitive experimental methods are required. This review presents a discussion of how state-of-the-art experimental protocols using ultrafast lasers and sensitive structural probes, such as electrons or X-rays are able to address this challenge. In particular, it is possible to investigate LSPC on single objects using single probe pulses and avoid accumulation effects in a heterogeneous sample. The presented results capture structure formation with femtosecond and atomic scale resolution. Ultrafast time-resolved probing approaches are key to revealing the transient states and pathways that govern material transformation in LSPC.

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.