Laser-matter Interaction Research Articles

Exploring fs-laser irradiation damage subthreshold behavior of dielectric mirrors via electrical measurements

Abstract With ultrafast laser systems reaching presently 10 PW peak power or operating at high repetition rates, research towards ensuring the long-term, trouble-free performance of all laser-exposed optical components is critical. Our work is focused on providing insight into the optical material behavior at fluences below the standardized laser-induced damage threshold (LIDT) value by implementing a simultaneous dual analysis of surface emitted particles using a Langmuir probe (LP) and the target current (TC). ${\mathrm{HfO}}_2$ and ${\mathrm{ZrO}}_2$ thin films deposited on fused silica substrates by pulsed laser deposition at various ${\mathrm{O}}_2$ pressures for defect and stoichiometry control were irradiated by Gaussian, ultrashort laser pulses (800 nm, 10 Hz, 70 fs) in a wide range of fluences. Both TC and LP collected signals were in good agreement with the existing theoretical description of laser–matter interaction at an ultrashort time scale. Our approach for an in situ LIDT monitoring system provides measurable signals for below-threshold irradiation conditions that indicate the endurance limit of the optical surfaces in the single-shot energy scanning mode. The LIDT value extracted from the LP-TC system is in line with the multipulse statistical analysis done with ISO 21254-2:2011(E). The implementation of the LP and TC as on-shot diagnostic tools for optical components will have a significant impact on the reliability of next-generation ultrafast and high-power laser systems.

Zeolite A with waste material: Morphological effect of laser treatments in air

The goal of this work is to investigate the action of femtosecond (fs) laser treatment performed under atmospheric conditions on morphology of zeolite A (LTA topology) synthesized with the addition of a waste material rich in iron such as red mud (RMFe-LTA) or introducing nanomagnetite (NMFe-LTA) during the hydrothermal synthesis. Both zeolite samples were analyzed by Scanning Electron Microscopy (SEM) technique. LTA zeolite formed using pure sources was assumed as reference. The results confirm that the presence of iron has a significant influence on laser-matter interactions affecting the morphology of the final treated synthetic material. Comparing to our previous data performed in vacuum conditions, these new results indicate that zeolite structure is generally preserved by fs laser treatment under atmospheric conditions although the presence of the Fe oxides distributed on the zeolite surfaces due to the use of red mud or in the form of nanomagnetite at specific locations, determines clear changes in treated zeolite morphology. However, the best performing results in terms of homogeneity of treatment and potential improvement of hydrophilic surface properties take places on zeolite formed with the addition of waste material (RMFe-LTA).

Change of dominant material properties in laser perforation process with high-energy lasers up to 120 kilowatt

In laser materials processing, energy losses due to reflection, heat conduction and thermal radiation play an important role. In this publication, we show that with increasing laser intensity, the energy lost within the sample becomes less important for metal perforation processes. We compare the laser-matter interaction of aluminum and steel plates. Material parameters such as density, melting point and especially thermal conductivity differ strongly, leading to much longer perforation times for aluminum in comparison to steel at laser powers of 20 kW. However, this behavior changes at laser powers of more than 80 kW where the perforation times of aluminum become shorter than the corresponding times for steel. By comparing experimental data and simulations, we conclude that thermal conduction is the dominant factor of energy loss at low powers, but is reduced at high laser powers.

Single Shot Generation of High‐Aspect‐Ratio Nano‐Rods from Sapphire by Ultrafast First Order Bessel Beam

AbstractEngineering the polarization and spatial phase of ultrafast laser pulses represents a compelling strategy for enhancing control over laser–matter interaction and enabling rapid and innovative nano‐fabrication processes. Here, the single‐shot, ultrafast laser fabrication of high‐aspect‐ratio, vertically standing nano‐pillars with a diameter of nm and height up to 15 µm on the surface of sapphire, is reported. To achieve this, the distinctive properties of diffraction‐free, first‐order Bessel beams endowed with either radial or azimuthal polarization distributions, are harnessed under tight focusing conditions. The highly intense laser–matter interaction in this configuration generates a tubular‐shaped, high‐pressure field beneath the material surface, leading to the rapid expulsion of material across the surface. Three distinct regimes for the pillar generation are identified in addition to a mechanism based on the Rayleigh‐Plateau theory that explains the distinct morphological regimes observed. The findings not only shed light on the underlying physical mechanisms of intense excitation of transparent dielectrics but also offer exciting prospects for the rapid fabrication of positive nano‐structures and material compression across various fields of application.

Laser-Based Selective Material Processing for Next-Generation Additive Manufacturing.

The connection between laser-based material processing and additive manufacturing is quite deeply rooted. In fact, the spark that started the field of additive manufacturing is the idea that two intersecting laser beams can selectively solidify a vat of resin. Ever since, laser has been accompanying the field of additive manufacturing, with its repertoire expanded from processing only photopolymer resin to virtually any material, allowing liberating customizability. As a result, additive manufacturing is expected to take an even more prominent role in the global supply chain in years to come. Herein, an overview of laser-based selective material processing is presented from various aspects: the physics of laser-material interactions, the materials currently used in additive manufacturing processes, the system configurations that enable laser-based additive manufacturing, and various functional applications of next-generation additive manufacturing. Additionally, current challenges and prospects of laser-based additive manufacturing are discussed.

Tunable spectral continuous shift of high-order harmonic generation in atoms by a plasmon-assisted shaping pulse

We delve into the phenomenon of high-order harmonic generation within a helium atom under the influence of a plasmon-assisted shaping pulse. Our findings reveal an intriguing manipulation of the frequency peak position in the harmonic emission by adjusting the absolute phase parameter within the frequency domain of the shaping pulse. This phenomenon holds potential significance for experimental setups necessitating precisely tuned single harmonics. Notably, we observe a modulated shift in the created harmonic photon energy, spanning an impressive range of 1.2 eV. This frequency peak shift is rooted in the asymmetry exhibited by the rising and falling edges of the laser pulse, directly influencing the position of the peak frequency emission. Our study quantifies the dependence of this tuning range and the asymmetry of the laser pulse, offering valuable insights into the underlying mechanisms driving this phenomenon. Furthermore, our investigation uncovers the emergence of semi-integer order harmonics as the phase parameter is altered. We attribute this discovery to the intricate interference between harmonics generated by the primary and secondary return cores. This observation introduces an innovative approach for generating semi-integer order harmonics, thus expanding our understanding of high-order harmonic generation. Ultimately, our work contributes to the broader comprehension of complex phenomena in laser-matter interactions and provides a foundation for harnessing these effects in various applications, particularly those involving precise spectral control and the generation of unique harmonic patterns.

Editorial: Advances and applications of laser-material interaction

Editorial: Advances and applications of laser-material interaction

Localized in-situ deposition: a new dimension to control in fabricating surface micro/nano structures via ultrafast laser ablation

Controllable fabrication of surface micro/nano structures is the key to realizing surface functionalization for various applications. As a versatile approach, ultrafast laser ablation has been widely studied for surface micro/nano structuring. Increasing research efforts in this field have been devoted to gaining more control over the fabrication processes to meet the increasing need for creation of complex structures. In this paper, we focus on the in-situ deposition process following the plasma formation under ultrafast laser ablation. From an overview perspective, we firstly summarize the different roles that plasma plumes, from pulsed laser ablation of solids, play in different laser processing approaches. Then, the distinctive in-situ deposition process within surface micro/nano structuring is highlighted. Our experimental work demonstrated that the in-situ deposition during ultrafast laser surface structuring can be controlled as a localized micro-additive process to pile up secondary ordered structures, through which a unique kind of hierarchical structure with fort-like bodies sitting on top of micro cone arrays were fabricated as a showcase. The revealed laser-matter interaction mechanism can be inspiring for the development of new ultrafast laser fabrication approaches, adding a new dimension and more flexibility in controlling the fabrication of functional surface micro/nano structures.Graphical

Laser material interactions in tamped materials on picosecond time scales in aluminum

A 100 ps laser is used to probe the pressure generation, depth of the non-solid ablator, and the non-linear optical effects through tamper materials. Samples consisted of an aluminum ablator with tampers of sapphire and coverslip glass. In general, the sapphire tamped sample achieves higher pressures at lower laser intensities as compared to the coverslip glass tamped sample. Attempts to model the details of this set of experimental data with standard available radiation coupled hydrodynamic codes make clear that more physics is needed in these simulations to accurately predict the impact of the tamper material on the pressure generation and the depth of non-solid aluminum.

Revealing the effects of laser beam shaping on melt pool behaviour in conduction-mode laser melting

Laser beam shaping offers remarkable possibilities to control and optimise process stability and tailor material properties and structure in laser-based welding and additive manufacturing. However, little is known about the influence of laser beam shaping on the complex melt-pool behaviour, solidified melt-track bead profile and microstructural grain morphology in laser material processing. A simulation-based approach is utilised in the present work to study the effects of laser beam intensity profile and angle of incidence on the melt-pool behaviour in conduction-mode laser melting of stainless steel 316L plates. The present high-fidelity physics-based computational model accounts for crucial physical phenomena in laser material processing such as complex laser–matter interaction, solidification and melting, heat and fluid flow dynamics, and free-surface oscillations. Experiments were carried out using different laser beam shapes and the validity of the numerical predictions is demonstrated. The results indicate that for identical processing parameters, reshaping the laser beam leads to notable changes in the thermal and fluid flow fields in the melt pool, affecting the melt-track bead profile and solidification microstructure. The columnar-to-equiaxed transition is discussed for different laser-intensity profiles.

Melt pool dynamics on different substrate materials in high-speed laser directed energy deposition process

High-speed laser directed energy deposition (HSL-DED) is a variant of the laser directed energy deposition process where a defocused metal powder stream is used, and it typically involves processing speeds exceeding 5 m/min. However, the interactions between the laser beam, powder stream, and substrate surface in HSL-DED have not been extensively studied. This study used a specialized XIRIS XVC-1000 welding camera with a narrow bandpass filter to record the interaction phenomenon. These observations were first carried out without powder delivery, using laser surface melting techniques, and involved processing speeds of up to 20 m/min and laser powers of up to 3 kW. HSL-DED with powder delivery was then conducted with the same parameter combinations for comparative analysis. The in situ observations in laser surface melting and HSL-DED identified a physical separation between the laser spot and the melt pool boundary, referred to as melt pool lag. Different substrates’ chemical compositions and the resulting thermophysical properties significantly impact melt pool dynamics during the high-speed laser-material interactions for a given process condition. The findings from this work have enabled a better understanding and control of melt pool dynamics in HSL-DED.

Enhancement of plasmonic coupling on Si metallized with intense femtosecond laser pulses

Using a pump–probe technique, the reflectivity of a silicon grating surface irradiated with intense femtosecond (fs) laser pulses was measured as a function of the incidence angle and the delay time between pulses. After irradiating the surface with an intense s-polarized, 400 nm, 300 fs laser pulse, the reflectivity measured with a weak p-polarized, 800 nm, 100 fs laser pulse exhibited an abrupt decrease for an incidence angle of ~ 24°. The depth of the dip was greatest for a delay time of 0.6–10 ps, for which the reflectivity around the dip was highest. The surface was also found to be ablated most strongly for the conditions causing the deepest dip for a delay time of 5–10 ps. Surface plasmon polaritons (SPPs) on silicon metallized by the intense pulse are resonantly excited by the subsequent pulse, and the strong coherent coupling between the subsequent pulse and SPPs excited on the molten Si surface produced by high-density free electrons induces strong surface ablation due to the intense plasmonic near-field. The results clearly show that fs pulses can be used to significantly modulate the nature of nonmetallic materials and could possibly serve as a basic tool for the excitation of SPPs on nonmetallic materials using ultrafast laser–matter interactions.

Differentiating multi-MeV, multi-ion spectra with CR-39 solid-state nuclear track detectors

The development of high intensity petawatt lasers has created new possibilities for ion acceleration and nuclear fusion using solid targets. In such laser-matter interaction, multiple ion species are accelerated with broad spectra up to hundreds of MeV. To measure ion yields and for species identification, CR-39 solid-state nuclear track detectors are frequently used. However, these detectors are limited in their applicability for multi-ion spectra differentiation as standard image recognition algorithms can lead to a misinterpretation of data, there is no unique relation between track diameter and particle energy, and there are overlapping pit diameter relationships for multiple particle species. In this report, we address these issues by first developing an algorithm to overcome user bias during image processing. Second, we use calibration of the detector response for protons, carbon and helium ions (alpha particles) from 0.1 to above 10 MeV and measurements of statistical energy loss fluctuations in a forward-fitting procedure utilizing multiple, differently filtered CR-39, altogether enabling high-sensitivity, multi-species particle spectroscopy. To validate this capability, we show that inferred CR-39 spectra match Thomson parabola ion spectrometer data from the same experiment. Filtered CR-39 spectrometers were used to detect, within a background of ~ 2 × 1011 sr−1 J−1 protons and carbons, (1.3 ± 0.7) × 108 sr−1 J−1 alpha particles from laser-driven proton-boron fusion reactions.

Online monitoring of local defects in robotic laser additive manufacturing process based on a dynamic mapping strategy and multibranch fusion convolutional neural network

Online monitoring is essential for laser additive manufacturing (AM) to improve in-process quality control. Currently, accurate monitoring of local defects in the laser AM process remains a challenge. This paper proposes a method for predicting local defects in the laser AM process based on a dynamic mapping strategy and the multibranch fusion convolutional neural network (MBFCNN). In-situ sensing of the laser-material interaction zone is achieved using a camera integrated coaxially with the printing system. Experiment-based datasets are constructed, in which the in-process images were sampled and matched to the extracted local defect information based on their temporal-spatial correspondence. A dynamic mapping strategy using a sliding sampling window is introduced to achieve continuous monitoring. Considering the cyclic and layer-by-layer processing principle of laser AM, we propose MBFCNN to map the in-process images to local defect information. A multibranch feature extraction module is designed based on the deposited layers of the target region to be monitored, in which each branch extracts high-dimensional representations from in-process images corresponding to a certain layer. We further introduce an attention mechanism to distinguish the importance of each branch and a feature fusion module to fuse the high-level information. Experimental results and comparison with traditional convolutional neural networks demonstrate the effectiveness of our method.

In situ metrology of direct-write laser ablation using optical emission spectroscopy

Direct-write laser ablation is an effective manufacturing method for etching complex microscale patterns, especially on hard ceramics such as sapphire that are difficult to machine using traditional mechanical or micromachining methods. However, the variability of the laser–matter interaction causes inconsistencies that prevent this process from moving beyond the research realm. This work presents the real-time monitoring of the ablation process in sapphire using optical emission spectroscopy to assess the key wavelengths that exhibit strong correlations to the fabricated features. In this process, a focused ultrafast laser is used to create microscale features and morphological changes in sapphire substrates, which are studied by a subsequent wet etching in a hydrogen fluoride solution. The etched sapphire samples are observed to have amorphous sapphire removed, resulting in microstructures with higher profile fidelity. Furthermore, principal component analysis of the measured spectral obtained during the etch process indicates that the emission from a few key wavelengths exhibits strong correlations to the etched sapphire patterns. This result indicates that the use of data-driven techniques to assess the spectral emissions of direct-write laser ablation can be a useful tool in developing in situ metrology methods for laser-matter interactions.

Generation of Light Fields with Controlled Non-Uniform Elliptical Polarization When Focusing on Structured Laser Beams

We study the sharp focusing of the input structured light field that has a non-uniform elliptical polarization: the parameters of the ellipse depend on the position in the input plane (we limited ourselves to the dependence only on the angular variable). Two types of non-uniformity were considered. The first type corresponds to the situation when the semi-axes of the polarization ellipse are fixed while the slope of the major semi-axis changes. The second type is determined by the situation when the slope of the major semi-axis of the polarization ellipse is constant, and the ratio between the semi-axis changes (we limited ourselves to the trigonometric dependence of this ratio on the polar angle). Theoretical and numerical calculations show that in the case of the first type of non-uniformity, if the tilt angle is a multiple of the polar angle with an integer coefficient, then the intensity distribution has rotational symmetry, and the energy flow is radially symmetric and has the negative direction near the optical axis. In this second case, the intensity symmetry is not very pronounced, but with an odd dependence of the ratio of the semi-axes of the polarization ellipse, the focused field at each point has a local linear polarization, despite the rather complex form of the input field. In addition, we investigate the distribution of the longitudinal component of the Poynting vector. The obtained results may be used for the formation of focused light fields with the desired distributions of polarization, Poynting vector density, or spin angular momentum density in the field of laser manipulation and laser matter interaction.

Developing system arrays for new experimental approach in nuclear astrophysics

The advent of facilities providing high-intensity and high-resolution gamma ray beams and/or ultra-short and high-repetition laser pulses can potentially open a new path of astrophysical research. Indeed, a pencil size gamma beams with tunable energies from few keV up to tens MeV will offer distinctive chances to conduct precise measurements of small cross sections (on the scale of μb or even smaller) pertaining to nuclear reactions in the field of astrophysics. Consequently, it provides essential data for modeling astrophysical S-factors crucial to stellar evolution. On the other hand, the possibility to mimic the stellar conditions by laser-matter interaction generating a controlled laboratory plasma with thermodynamical status not too different from stellar conditions will open the way for the study of nuclear reactions of utmost importance for nuclear astrophysics.For photonuclear reactions with astrophysical significance, as photodissociations occur at photon energies slightly above particle emission thresholds due to typical stellar temperatures, the resulting fragments possess low energies spanning from a few hundred keV to a few MeV. Consequently, detectors with low thresholds become imperative in such cases. Also, in the case of laser-induced reactions, in order to detect the fusion products and to measure the laser-accelerated ion distribution a proper system of detection is needed. Depending on the available exit channels of the nuclear reaction of interest, both charged particles and neutrons are foreseen.Here, we present the Asfin’s efforts on developing new detectors arrays suitable for the experimental requirements in these challenging measurements. Indeed, an experimental campaign is ongoing in order to test the feasibility of excitation functions and angular distributions determinations using versatile silicon strip arrays (namely LHASA and/or ELISSA). Moreover, extensive studies and simulations will be presented regarding the developing of a dedicated detection system comprising a cryogenically cooled supersonic nozzle, an appropriate interaction chamber, an array of neutron and charged particle detectors and two compact ion spectrometers for performing systematic study of laser-induced nuclear fusion reactions.

A Review of Computational Approaches to the Microstructure-Informed Mechanical Modelling of Metals Produced by Powder Bed Fusion Additive Manufacturing.

In the rapidly evolving field of additive manufacturing (AM), the predictability of part properties is still challenging due to the inherent multiphysics complexity of the technology. This results in time-consuming and costly experimental guess-and-check approaches for manufacturing each individual design. Through synthesising advancements in the field, this review argues that numerical modelling is instrumental in mitigating these challenges by working in tandem with experimental studies. Unique hierarchical microstructures induced by extreme AM process conditions- including melt pool patterns, grains, cellular-dendritic substructures, and precipitates-affect the final part properties. Therefore, the development of microstructure-informed mechanical models becomes vital. Our review of numerical studies explores various modelling approaches that consider the microstructural features explicitly and offers insights into multiscale stress-strain analysis across diverse materials fabricated by powder bed fusion AM. The literature indicates a growing consensus on the key role of multiscale integrated process-structure-property-performance (PSPP) modelling in capturing the complexity of AM-produced materials. Current models, though increasingly sophisticated, still tend to relate only two elements of the PSPP chain while often focusing on a single scale. This emphasises the need for integrated PSPP approaches validated by a solid experimental base. The PSPP paradigm for AM, while promising as a concept, is still in its infantry, confronting multifaceted challenges that require in-depth, multidisciplinary expertise. These challenges range from accounting for multiphysics phenomena (e.g., advanced laser-material interaction) and their interplay (thermo-mechanical and microstructural evolution for simulating Type II residual stresses), accurately defined assumptions (e.g., flat molten surface during AM or purely epitaxial solidification), and correctly estimated boundary conditions for each element of the PSPP chain up to the need to balance the model's complexity and detalisation in terms of both multiphysics and discretisation with efficient multitrack and multilayer simulations. Efforts in bridging these gaps would not only improve predictability but also expedite the development and certification of new AM materials.

Influence of the spatial laser energy absorption on the molten pool dynamics in high-power laser beam welding

The spatial laser energy absorption inside the keyhole is decisive for the dynamic molten pool behaviors and the resultant weld properties in high-power laser beam welding (LBW). In this paper, a numerical simulation of the LBW process, considering the 3D transient heat transfer, fluid flow, and keyhole dynamics, is implemented, in which the free surface is tracked by the volume-of-fluid algorithm. The underlying laser-material interactions, i.e., the multiple reflections and Fresnel absorption, are considered by an advanced ray-tracing method based on a localized level-set strategy and a temperature-dependent absorption coefficient. The laser energy absorption is analyzed from a time-averaged point of view for a better statistical representation. It is found for the first time that a significant drop in the time-averaged laser energy absorption occurs at the focus position of the laser beam and that the rest of the keyhole region has relatively homogeneous absorbed energy. This unique absorption pattern may lead to a certain keyhole instability and have a strong correlation with the detrimental bulging and narrowing phenomena in the molten pool. The influence of different focus positions of the laser beam on the keyhole dynamics and molten pool profile is also analyzed. The obtained numerical results are compared with experimental measurements to ensure the validity of the proposed model.

Four-framed X-ray imaging crystal spectrometer for time-resolved laser plasma diagnostics

X-ray imaging crystal spectrometers with high spectral and spatial resolutions are the important diagnostic tools for acquiring high-quality plasma information in the study of laser–matter interactions. In this paper we propose a new multi-channel imaging crystal spectrometer with coupling X-ray framing camera to record the dynamic evolution of a laser-induced plasma: a four-channel imaging mica crystal spectrometer with a spatial resolution of 40–60 μm for diagnosing the aluminum (Al) plasma targets was developed. Calibration experiments revealed that the spectral resolution is higher than 1700 at Al Kα3 characteristic lines (1496.4eV). Further, we established an aiming method based on the positioning ball and two monitoring CCDs. The optical design, mechanical design, and experimental measurements of the four-channel imaging mica crystal spectrometer are discussed.