Laser-induced Modification Research Articles

Ultrafast imaging of laser-induced modifications in planar targets via single-shot in-line holography

Abstract We present a holographic single-shot coherent diffractive imaging method based on in-line holography that allows the ultrafast characterization of 2D transmission maps of semi-transparent planar targets such as foils in amplitude and phase. Holographic information is obtained from the interference of the transmitted primary beam with the fields scattered from the modified or unknown target regions. A specialized iterative phase retrieval is used to incorporate the holographic nature of the approach and to accelerate and improve convergence. The achievable quality and reproducibility of the reconstructed transmission maps as well as optimal setup parameters are investigated using realistic pre-characterized reference targets. We used non-circular laser-induced hole structures in 30 nm thin gold foils that represent the final state of a laser modification and show that the far field error of the reconstructed diffraction images can be used to estimate and optimize the reconstruction quality in the object plane in order to obtain accurate and reproducible transmission maps. Our results mark the important first step towards the full spatio-temporal analysis of all stages of laser material modification or laser ablation in two-color pump probe experiments, including

A New Approach Toward Extreme Thermal Stability of Femtosecond Laser Induced Modifications in Glasses

AbstractImprinting thermally stable transformations by femtosecond laser in glass would benefit the development of optical sensors dedicated to harsh environments including combustors, nuclear reactors, aircraft engines, or metal/ceramic manufacturing processes. While glass brings undeniable assets over refractory crystalline materials like shaping ability (e.g., optical fiber form), one key challenge is to prevent the erasure of induced transformations at high temperatures and for long periods. In this article, the role of glass composition and viscosity to achieve modifications stable at high temperatures is first reviewed, providing a comprehensive roadmap for engineers in optics and photonics. While silica appears to be the candidate of choice, it is revealed that binary aluminosilicates can compete and sometimes surpass it. The hypothesis is formulated and investigated that a hybrid glass‐crystalline nano‐structuring can imprint ultra‐stable modifications inside glass. Laser‐induced modifications in Al2O3‐SiO2 and ZrO2‐Al2O3‐SiO2 glasses reveal a partial crystallization, shaped into a lamellar structure and orientable with laser light polarization. These birefringent structures can withstand temperatures up to 1300 °C for 30 minutes. Even after erasure, a positive index contrast persists, up to 1650 °C for binary 60Al2O3‐40SiO2 (mol%). This is the first observation of this kind of persisting index contrast, paving the way to ultra‐stable glass‐based optical waveguiding.

Femtosecond laser-induced damage threshold incubation in SrTiO3 thin films

This study delved into the microfabrication process on SrTiO3 thin films using femtosecond laser pulses, focusing on characterizing ablation behavior and elucidating on incubation parameters. A combination of microscopy techniques and systematic analysis of laser-material interactions provided insights into the features and fundamental processes governing the laser-induced modification of SrTiO3 thin films. The cutting profile morphology indicated high-resolution material removal, with line widths ranging from 0.6 to 1.6 µm, depending on number of pulses and energy. The study revealed a low incubation parameter for SrTiO3 thin films, quantified as (0.03 ± 0.01), indicating that a considerable number of pulses is required to cause damage to the material. Additionally, threshold energies for damage were determined to be 17.9 nJ for 20,000 pulses and 35.2 nJ for a single pulse. AFM analysis showed that the depth of the micromachined lines increased from approximately 80 nm to 315 nm with higher pulse energies, confirming substantial material removal. The Keldysh parameter provides insights into ionization mechanisms under femtosecond-laser irradiation, highlighting the dominance of tunneling ionization. Using higher pulse energies, the center of the microstructures exhibited significantly reduced Raman signals, correlating with substantial material removal, which was confirmed by atomic force microscopy. EDS analyses indicated that the elemental composition of the irradiated regions closely resembled the stoichiometric composition of SrTiO3, while the center of the microstructures showed significant oxygen levels but lacked titanium and strontium, consistent with the findings from AFM and Raman microscopy.

Enhanced osteointegration and osteogenesis of osteoblast cells by laser-induced surface modification of Ti implants

Dental and orthopedic implants have become routine medical technologies for tooth replacement and bone fixation. Despite significant progress in implantology, achieving sufficient osseointegration remains a challenge, often leading to implant failure over the long term. Nanotechnology offers the potential to mimic the natural patterns of living tissues, providing a promising platform for tissue engineering and implant surface design. Among the various methods for developing nanostructures, High-Regular Laser-Induced Periodic Surface Structures (HR-LIPSS) techniques stand out for their ability to fabricate highly ordered nanostructures with excellent long-range repeatability and production efficiency. In this study, we utilized an innovative technical approach to generate traditional laser-induced superficial LIPSS nanostructures, followed by detailed surface analysis using classical microscopy and physicochemical methods. Our findings demonstrate for the first time that nanostructured LIPSS surfaces can significantly enhance cell adhesion and proliferation while providing an optimal environment for cell metabolism. Given the high reproducibility, low cost, and potential of HR-LIPSS techniques to support cell growth and differentiation, this novel technology has the potential to impact both the industrial development of new implants and clinical outcomes after implantation.

Fabricating ultra wear-resistant surfaces on titanium alloy by combining laser-induced modification with abrasive belt grinding

A novel method combining laser-induced modification and abrasive belt grinding (LM&BG) was proposed to prepare wear-resistant surfaces of titanium alloys. Comprehensively investigated the evolution mechanism of material modification and wear resistance of LM&BG samples. It was found that an oxide-ceramic-modified layer with increasing hardness (10 times) was formed by laser modification on the titanium alloy, mainly composed of TiO2, with the two phases Rutile-TiO2 and Anatase-TiO2. Moreover, the surface roughness after abrasive belt grinding of the modified layer is less than Ra0.1, with a uniform subsurface microstructure. Interestingly, the wear modes of the modified layer shifted from adhesive and oxidative wear to brittle microdetachment, significantly reducing the coefficient of friction, material wear, and subsurface damage, thus improving the wear-resistant capability.

Study of damage mechanism on single crystal 4H-SiC surface layer by picosecond laser modification (PLM)

The stable structure, high hardness, and brittleness of single-crystal 4H-SiC present challenges in achieving efficient and damage-free polishing processing. Ultra-short pulsed laser-induced surface modification provides a new solution to enhance the manufacturability of 4H-SiC. This paper aims to investigate the underlying mechanism of the picosecond laser-induced surface structural changes in 4H-SiC and the process of material removal from solid to vapour, elucidating the interaction mechanism between picosecond laser and 4H-SiC. A temperature gradient distribution model for picosecond laser irradiation on 4H-SiC was built to reveal the formation mechanism of subsurface crack damage. The results show that a combination of phase explosion and thermal effects controlled picosecond laser-modified 4H-SiC surfaces deposited spherical SiO2 particles and the process. The study demonstrates that the cracking behaviour in the subsurface of picosecond laser-modified SiC predominantly occurs in the recast region. The fundamental cause of cracking is attributed to the tensile stresses generated by thermal effects and the volumetric changes in the molten material resulting from the overlapping of neighbouring spots in alternating hot–cold cycles. The nanoindentation demonstrated that laser modification can effectively enhance the machinability of SiC. The findings of this study can provide a theoretical basis for efficient non-destructive machining of hard and brittle materials.

Bessel beam fabrication of graphitic micro electrodes in diamond using laser bursts

We present the fabrication of conductive graphitic microelectrodes in diamond by using pulsed Bessel beams in the burst mode laser writing regime. The graphitic wires are created in the bulk of a 500 μm thick monocrystalline HPHT diamond (with (100) orientation) perpendicular to the sample surface, without beam scanning or sample translation. In particular, the role of different burst features in the resistivity of such electrodes is investigated for two very different sub-pulse durations namely 200 fs and 10 ps, together with the role of thermal annealing. Micro-Raman spectroscopy is implemented to investigate the laser-induced crystalline modification, and the results obtained by using two different laser repetition rates, namely 20 Hz and 200 kHz, are compared. A comparison of the micro-Raman spectra and of the resistivity of the electrodes fabricated respectively with 10 ps single pulses and with bursts (of sub-pulses) of similar total duration has also been made, and we show that the burst mode writing regime allows to fabricate more conductive micro electrodes, thanks to the heat accumulation process leading to stronger graphitization. Moreover, the microfabrication of diamond by means of the longest available bursts (~ 46.7 ps duration) featured by 32 sub-pulses of 200 fs duration, with intra-burst time delay of 1.5 ps (sub-THz bursts), leads to graphitic wires with the lowest resistivity values obtained in this work, especially at low repetition rate such as 20 Hz. Indeed, micro electrodes with resistivity on the order of 0.01 Ω cm can be fabricated by Bessel beams in the burst mode regime even when the bursts are constituted by femtosecond laser sub-pulses, in contrast with the results of the standard writing regime with single fs pulses typically leading to less conductive micro electrodes.

Controllable Si micro-structuring by temporally modulated single-shot femtosecond pulse lithography

Micro-structured silicon surface plays a significant role in the electronics industry. Crystalline and amorphous phases of silicon, which have significantly different physical properties, can be transformed into each other using femtosecond laser, and assisted chemical etching enables the easy fabrication of various micro-structures. However, efficient and controllable fabrication of micro-cylinders, micro-rings, and micro-ring grooves on silicon surfaces still remains a challenge. Here, we propose a temporally modulated single-shot femtosecond pulse lithography technology, combining laser-induced silicon modification and chemical etching. In this technology, the occurrence of recrystallization allows for the flexible manipulation of the shape of the modified area, transitioning from circular to annular shapes by adjusting the laser fluence. This corresponds to the formation of micro-cylinders and micro-rings after etching. In the case of temporally shaped pulses, with an increasing pulse delay, the micro-rings formed after etching gradually transform into micro-ring grooves, as the recrystallization area created by the first sub-pulse is reacted by the second sub-pulse. Due to the characteristics of the single shot, the technology, when combined with the flying punch method, can be used for the high-efficiency fabrication of large-area silicon surface micro-structure arrays.

Energy-dependent correlation between micro/nano-morphology and stress state of femtosecond laser-induced modification in fused silica

Due to the importance of stress control in the laser processing of brittle transparent dielectrics such as glass and crystal, investigating the evolution of laser-induced stress state and its manipulation mechanism is highly desirable for ensuring high-performance crack-free laser micro/nanostructuring of dielectrics. Herein, the internal stress state of a femtosecond laser-modified fused silica glass sample at a broad range of pulse energy from 0.34 to 3.5 μJ was evaluated using a cantilever displacement method. Two obvious transition zones in the variation of microscale displacement were identified and analyzed by polarized optical microscopy, scanning electronic microscopy, and confocal micro-Raman spectroscopy. A strong energy-dependent correlation between the evolution of the micro/nano-scale morphology and stress state in laser-induced structures has been investigated and discussed. The analyzed result is beneficial for developing high-efficiency and high-quality manufacturing of 3D large-scale and high-precision glass microstructures.

Dominant role of laser-generated nano-structures on enhancement of interfacial bonding strength by laser surface modification

Laser surface modification has been widely applied to the pre-treatment of material surfaces for interface bonding, however, laser-treated surfaces face an inevitable risk of environmental exposure during engineering application, and it remains unclear how laser-induced modifications change under environmental exposure. This study investigates the robustness of laser-treated aluminum surface under typical atmospheric and aqueous environments at different temperature from −40 °C to 80 °C. Although the carbon elemental content and water contact angle greatly fluctuate with exposure environments from 23 at.% to 33 at.%, and from 0° to 148°, respectively, laser-treated surfaces exhibit stable improvement of ∼25 % in bonding strength compared to untreated condition even after 30 days exposure. Notably, hygrothermal exposure of 80 °C and 95 %RH significantly declines bonding strength by ∼45 %, and this strength degradation is accompanied by 1.8 % increase of carbon elemental content and water contact angle of ∼150°. Results reveal that laser-generated nano-structures play crucial role in strength improvement, instead of carbonaceous contaminants and surface polarity, and nanostructure transformation causes an apparent strength degradation and change of fracture mode from cohesive to interface. This work uncovers the dominant role of laser-generated nano-structures in enhancing interfacial bonding strength, and provides supports for engineering application of laser surface modification.

Investigations on friction stir composite processing and picosecond laser-induced surface modification on mechanical properties of β-titanium alloy

This work introduced a new surface modification method of titanium alloy Ti-35Nb-2Ta-3Zr alloy by friction stir machining (FSP) and picosecond laser-induced surface modification (PsLSM). The results show that the properties of the alloy are the best when the picosecond laser scanning speed is 50 mm/s, the laser power is 0.5 W. Compared with the untreated, the microhardness is increased by 68.7%, Young's modulus is reduced by 40%, the friction coefficient is reduced by 81.8%, and the friction loss is reduced by 62.5%. Among them, the experimental results show that the effect of PsLSM on performance improvement is higher than that of FSP, but the performance of samples treated with FSP+PsLSM is higher than that of samples treated with PsLSM. It was found that the LIPSS formed on the sample's surface after PsLSM treatment is the main reason for the improvement of mechanical properties, and the ultra-high temperature remelting effect of FSP treatment makes the surface grains refined. This study provides a new idea and method for surface modification of β titanium alloy to improve its mechanical properties.

Evaluation of microscale crystallinity modification induced by laser writing on Mn3O4 thin films

Defining microstructures and managing local crystallinity allow the implementation of several functionalities in thin film technology. The use of ultrashort Bessel beams for bulk crystallinity modification has garnered considerable attention as a versatile technique for semiconductor materials, dielectrics, or metal oxide substrates. The aim of this work is the quantitative evaluation of the crystalline changes induced by ultrafast laser micromachining on manganese oxide thin films using micro-Raman spectroscopy. Pulsed Bessel beams featured by a 1 μm-sized central core are used to define structures with high spatial precision. The dispersion relation of Mn3O4 optical phonons is determined by considering the conjunction between X-ray diffraction characterization and the phonon localization model. The asymmetries in Raman spectra indicate phonon localization and enable a quantitative tool to determine the crystallite size at micrometer resolution. The results indicate that laser-writing is effective in modifying the low-crystallinity films locally, increasing crystallite sizes from ∼8 nm up to 12 nm, and thus highlighting an interesting approach to evaluate laser-induced structural modifications on metal oxide thin films.

Study on the effect of laser-induced oxidation on color change and hydrophilicity of 2D codes

The effect of laser-induced oxidation on color change and surface wettability of anti-oil and super-hydrophobic two-dimensional code (2D Code) is demonstrated, where the color change and wettability are very important to the reading quality and durability of 2D Code. The poor oleophobicity and hydrophobicity, which render laser direct part marking of 2D Code inadequate for application in oil pollution, corrosion environment, are greatly improved with combination method of accurate laser-induced oxidation and surface modification. To obtain the control mechanism for the color change and wettability of 2D Code, the influence of laser processing parameters on the oxidation and microstructure change of the modules laser-marked on titanium alloy were studied, and then the influence of oxidation, microstructure and surface modification processing parameters on the hydrophilicity of 2D Code were analyzed. From the experimental results, with the laser energy density increasing, the oxidation degree increases and the modules show platinum grey, violet blue, graphite black, light ivory, jet black and other color states, with its hydrophilicity of the modules laser processed becoming stronger first and weaker later. The modules also present various microstructures with different laser processing parameters, such as pits, cracks, wormlike protrusions, which also have an important effect on the color change and wettability of modules. By laser parameters optimizing, it is found that when laser power is 10 W, Q frequency 45 KHZ, scanning speed 200 mm/s and hatching spacing 0.03 mm, the 2D code with high contrast and hydrophilic oleophylic property was obtained, where the static water contact angle could be a minimum to about 6.0° and the static oil contact angle could be a minimum to 8.1°. After the surface modification processing, where a coating consisting of heptadecafluorotrichlorosilane and n-heptanea with specific ratio is sprayed by special process on the surface, the 2D code show anti-oil and super-hydrophobic property, within the static water contact angle being a maximum to about 178.7° and the static oil contact angle being a maximum to 132.8°. In addition, it is found that the stronger hydrophilicity of modules laser processed before coating spraying, the stronger hydrophobicity of 2D code after coating spraying shall show.

Diffusion of muonic hydrogen in hydrogen gas and the measurement of the 1$s$ hyperfine splitting of muonic hydrogen

The CREMA collaboration is pursuing a measurement of the ground-state hyperfine splitting (HFS) in muonic hydrogen (\muμp) with 1 ppm accuracy by means of pulsed laser spectroscopy. In the proposed experiment, the \muμp atom is excited by a laser pulse from the singlet to the triplet hyperfine sub-levels, and is quenched back to the singlet state by an inelastic collision with a H_22 molecule. The resulting increase of kinetic energy after this cycle modifies the \muμp atom diffusion in the hydrogen gas and the arrival time of the \muμp atoms at the target walls. This laser-induced modification of the arrival times is used to expose the atomic transition. In this paper we present the simulation of the \muμp diffusion in the H_22 gas which is at the core of the experimental scheme. These simulations have been implemented with the Geant4 framework by introducing various low-energy processes including the motion of the H_22 molecules, i.e. the effects related with the hydrogen target temperature. The simulations have been used to optimize the hydrogen target parameters (pressure, temperatures and thickness) and to estimate signal and background rates. These rates allow to estimate the maximum time needed to find the resonance and the statistical accuracy of the spectroscopy experiment.

Design of Surfaces with Persistent Antimicrobial Properties on Stainless Steel Developed Using Femtosecond Laser Texturing for Application in "High Traffic" Objects.

Metal-based high-touch surfaces used for diverse applications in everyday use, like handrails, playground grab handles, doorknobs, ATM touch pads, and desks, are the most common targets for pollution with a variety of microbes; there is thus a need to improve their antimicrobial properties, an issue which has become a challenge in recent years, particularly after the COVID-19 pandemic. According to the World Health Organization (WHO), drug-resistant pathogens are one of the main concerns to global health today, as they lead to longer hospital stays and increased medical costs. Generally, the development of antimicrobial surfaces is related to the utilization of chemical methods via deposition on surfaces in the forms of various types of coatings. However, the addition of chemical substances onto a surface can induce unwanted effects, since it causes surface chemistry changes and, in some cases, cannot provide long-lasting results. A novel approach of utilising ultra-short laser radiation for the treatment of metallic surfaces by inducing a variety of micro- and nanostructuration is elaborated upon in the current research, estimating the optimum relation between the wettability and roughness characteristics for the creation of antimicrobial properties for such high-touch surfaces. In the current study, AISI 304-304L stainless steel metal was used as a benchmark material. Surface texturing via laser ablation with femtosecond laser pulses is an effective method, since it enables the formation of a variety of surface patterns, along with the creation of bimodal roughness, in one-step processing. In this investigation, a precise approach toward developing hydrophobic stainless steel surfaces with tunable adherence using femtosecond laser-induced modification is described. The impact of basic femtosecond laser processing parameters, like the scanning velocity, laser energy, and wettability properties of the laser-processed stainless steel samples, are examined. It is identified that the topography and morphology of laser-induced surface structures can be efficiently changed by adapting the laser processing parameters to create structures, which facilitate the transfer of surface properties from extremely low to high surface wettability.

Neural network interatomic potential for laser-excited materials

Data-driven interatomic potentials based on machine-learning approaches have been increasingly used to perform large-scale, first-principles quality simulations of materials in the electronic ground state. However, they are not able to describe situations in which the electrons are excited, like in the case of material processing by means of femtosecond laser irradiation or ion bombardment. In this work, we propose a neural network interatomic potential with an explicit dependency on the electronic temperature. Taking silicon as an example, we demonstrate its capability of reproducing important physical properties with first-principles accuracy and use it to simulate laser-induced surface modifications on a thin film at time and length scales that are impossible to reach with first-principles approaches like density functional theory. The method is general and can be applied not only to other laser-excited materials but also to condensed and liquid matter under non-equilibrium situations in which electrons and ions exhibit different temperatures.

Surface Modification of Brass via Ultrashort Pulsed Direct Laser Interference Patterning and Its Effect on Bacteria-Substrate Interaction.

In recent decades, antibiotic resistance has become a crucial challenge for human health. One potential solution to this problem is the use of antibacterial surfaces, i.e., copper and copper alloys. This study investigates the antibacterial properties of brass that underwent topographic surface functionalization via ultrashort pulsed direct laser interference patterning. Periodic line-like patterns in the scale range of single bacterial cells were created on brass with a 37% zinc content to enhance the contact area for rod-shaped Escherichia coli (E. coli). Although the topography facilitates attachment of bacteria to the surface, reduced killing rates for E. coli are observed. In parallel, a high-resolution methodical approach was employed to explore the impact of laser-induced topographical and chemical modifications on the antibacterial properties. The findings reveal the underlying role of the chemical modification concerning the antimicrobial efficiency of the Cu-based alloy within the superficial layers of a few hundred nanometers. Overall, this study provides valuable insight into the effect of alloy composition on targeted laser processing for antimicrobial Cu-surfaces, which facilitates the thorough development and optimization of the process concerning antimicrobial applications.

Nanosecond laser-induced filamentous modification in silicate glasses

Laser-induced filamentous modification (LIFM) has been extensively studied in transparent materials and is widely used for cutting and drilling with a high aspect ratio. However, the discussion focuses on the domain of ultra-short pulses recently, such as picosecond and femtosecond pulses. In this work, a LIFM by a short pulse in silicate glasses is demonstrated, and the physical mechanism is investigated. In the soda-lime glass, a filamentous modification with a length of >650 μm was introduced by a nanosecond pulse with an energy of 0.86 mJ. The average length of LIFM is positively correlated with the pulse energy but is not affected by the focus depth significantly. Furthermore, LIFM is also observed in borosilicate glass and K9 glass, but not in fused silica glass. Theoretical analysis shows that the contribution of the self-focusing and optical error to LIMB could be ignored in soda-lime glass. An improved Laser-induced moving breakdown (LIMB) model is proposed to describe LIFM by a nanosecond pulse, and the thermal explosion (TE) model is used to explain the micro-structures. High-density defects dominate the LIMB of nanosecond pulses in silicate glasses, which provide seed electrons for laser-induced ionization and induce a thermal explosion through enhanced absorption. The estimation result of the TE model shows that the final temperature of thermal plasma exceeds 1 × 105 K, resulting in micro-explosion and forming micro-voids and micro-crack clusters. Based on the principle of LIFM, stealth cutting was implemented by a high-frequency nanosecond laser in soda-lime glass with a thickness of 2 mm, and the cutting speed reaches 3000 mm/s.

Third-harmonic generation monitoring of femtosecond-laser-induced in-volume functional modifications

During the last two decades, ultrafast in-volume laser-based processing of transparent materials has emerged as a key 3D-printing method for manufacturing a variety of complex integrated photonic devices and micro-parts. Yet, identifying suitable laser process parameters for a given substrate remains a tedious, time-consuming task. Using a single laser source for both processing and monitoring, we demonstrate a method based on in situ full-field third-harmonic generation (THG) microscopy that exploits the properties of a low-noise CMOS imager to rapidly identify the entire processing space, discriminating different types of laser-induced modifications, and extracting incubation laws governing the laser exposure process. Furthermore, we show that full-field THG monitoring is capable of identifying parameters leading to enhanced functional properties, such as laser-enhanced etching selectivity. These findings enable accelerated implementations of laser processes of arbitrarily chosen transparent materials and, due to the rapid acquisition time (>100FPS) of the imager, closed-loop process control.

Modification of Vibrational Parameters of a CO2 Molecule by a Laser Field: Impact on Tunnel Ionization

In this paper, we theoretically study the laser-induced modification of the vibrational parameters of a carbon dioxide molecule regarding its tunnel ionization. Our study predicts a 5% increase in the ionization rate in anti-Stokes channels that corresponds to pumping the Σu mode up to vai=10. The molecule is imparted with an additional energy from the pre-pumped vibrational states, which is absorbed during ionization. As a result, the tunneling rate increases. This amplification of tunnel ionization of the CO2 gas target can potentially be used for the laser separation of carbon isotopes.