László Égerházi

We demonstrate a single-shot method for probing ultrafast laser–matter interactions using unfocused multi-terawatt femtosecond beams. The intrinsic fluence inhomogeneity of the beams is exploited to accomplish parallel, spatially encoded experiments performed under strictly identical conditions across a wide energy range in a single step, enabling systematic mapping of energy-dependent surface responses without beam scanning. As proof of the technique, the large set of simultaneously collected data points below, at, and above the threshold inherently allows for a threshold evaluation approach, different from both the diameter/depth regression analysis and the statistical method. Using copper as a test system, we quantify the fluence dependence of reflectance and morphology and identify a multi-pulse optical threshold of 0.020 J/cm2, coinciding with the onset of ablation confirmed by SEM imaging. The minimum specular reflectance observed, around 0.10%, favorably compares to the respective figures reported. Applying this parallelized surface mapping technique with high-energy laser systems featuring suitably large beam diameters thereby provides a versatile platform for exploring ultrafast laser-induced surface responses, with potential applications in material design, surface engineering, and optical damage studies.

Wire explosion (WE) inherently generates particle ensembles spanning the nano- to microscale, posing challenges for conventional characterization methods in terms of capturing the full particle population. To address this issue, spectrophotometric analysis combined with algorithmic spectrum reconstruction based on Mie theory and constrained distribution models were employed to characterize copper WE products formed in aqueous surroundings within the 4–12 kV discharge voltage range. Three independent fitting strategies, specifically a semimanual fitting, an evolutionary algorithm, and a grid search, were applied to retrieve the size distributions and relative shares of copper and copper oxide particles as a function of discharge voltage. Based on experimental and theoretical findings, lognormal and normal distributions across the 10–300 nm diameter range were assumed as constraints for oxide and metallic fractions, respectively. The reconstructed metallic copper population exhibited mean diameters ranging from 123 to 181 nm, while oxidized fractions followed lognormal distributions centred near 10 nm mode diameters. Voltage-dependent trends revealed an optimal discharge regime between 6 kV and 8 kV, where the exploded fraction reached approximately 63% and the metallic mass share exceeded 80%. These results confirmed that spectrophotometry represents an essential tool for the quantitative characterization of such complex, wide-range systems.

Aquasols produced by exploding copper wires represent complex systems in which identifying individual colloidal components poses challenges due to broad and multimodal size distributions and varying shares among oxidation states. To evaluate the reproducibility of copper wire explosion, the size distribution of metallic and oxidized colloidal components within the 10–300 nm diameter range was assessed. Classification of each individual particle into bins according to size and chemical composition was accomplished by reconstructing the recorded optical extinction spectra of three sols produced under identical conditions as the weighted sum of the extinction spectra of individual copper and copper-oxide particles, computed using Mie theory. Our spectrophotometry-based component analysis revealed differences in particle number concentrations of the mainly oxidized nanoparticles, corresponding to deviations observed in the ultraviolet portion of the extinction spectra. Notable uniformity was observed, however, in the number of metallic fine particles, consistent with agreement in spectral features in the visible range. Regarding mass concentration, practically no differences were observed among the three samples, with nano-to-fine ratios of copper particles agreeing within 0.45%. Despite the complex processes during explosion leading to limited reproducibility in the ratio of different copper oxidation states, very good reproducibility (54.2 ± 0.7%) was found when comparing the total copper content of the samples to the mass of the exploded copper wire.

The availability of new-generation femtosecond lasers capable of delivering pulses with energies in the hundreds of mJ, or even in the joules range, has called for a revision of the effect of scaling spot size on the material distribution within the plasma plume. Employing a state-of-the-art Szatmári-type hybrid dye-excimer laser system emitting 248 nm pulses with a maximum energy of 20 mJ and duration of 600 fs, copper films were grown in the classical pulsed laser deposition geometry. The exceptionally clean temporal profile of the laser pulses yielded a femtosecond component of 4.18 ± 0.19 mJ, accompanied by a 0.22 ± 0.01 mJ ASE pedestal on the target surface. While varying the spot sizes, the plasma plume consistently exhibited an extremely forward-peaked distribution. Deposition rates, defined as peak thickness per number of pulses, ranged from 0.030 to 0.114 nm/pulse, with a gradual narrowing of the thickness distribution as the spot area increased from 0.085 to 1.01 mm² while keeping the pulse energy constant. The material distribution on the silicon substrates was characterized using the f(Θ) = Acos^kΘ + (1 − A)cos^pΘ formalism, revealing exponents characterizing the forward-peaked component of the thickness profile of the film material along the axes, ranging from k = 15 up to exceptionally high values exceeding 50, as the spot area increased. Consequently, spot size control and outstanding beam quality ensured that majority of the ablated material was confined to the central region of the plume, indicating the potential of PLD (pulsed laser deposition) for highly efficient localized deposition of exotic materials.

Wire explosion is a technically straightforward green method for nanoparticle production; however, the determination of polydisperse and multimodal particle size distribution and thereby the assessment of the efficiency of the process pose serious challenges. Fitting extinction spectra derived from the Mie theory to the measured extinction spectra of colloidal solutions produced by the explosion of copper wires of 70 μm diameter and 20 mm length by discharging a capacitor of 435 nF charged to voltages ranging from 4 to 12 kV in an aqueous environment allows for determining the size distribution of the particles in the 10–300 nm diameter range and provides information on their oxidation state as well. In underwater wire explosion of copper, the vast majority of the nanoparticles transforms into copper oxides already during the process. The size distribution remains bimodal throughout the whole charging voltage range investigated. The nanoparticles follow a lognormal size distribution with a mode at 30 nm, while the fine particle population extending from 100 to 300 nm in diameter is dominated by rather normally distributed copper droplets with modes at 160–170 nm. The energy injected into the wire controls the individual concentration of both the fine and nanoparticles with the modes and ranges remaining hardly affected. The number concentration of the nanoparticles increases monotonically with the energy, spanning more than one order of magnitude. The throughput assessed by the total mass of the nanoparticles divided by the total mass of all species identified in the sol within the 0–300 nm diameter range increases monotonically from 1% at 4 kV up to 41% at 12 kV, revealing that wire explosion can be optimized for efficient nanoparticle production.