Ultrashort Laser Pulses Research Articles

Influence of Oxide Formation Following Ultrashort Pulsed Laser Micromachining on Self‐Propagating Reactions in Free‐Standing Ni/Al Reactive Multilayer Foils

Reactive metallic multilayers, renowned for their exothermic self‐propagating reactions, show a distinct ability to achieve minimal thermal influence in processes such as welding and soldering, where minimizing the thermal impact on the material being joined is crucial. Ultrashort pulsed lasers offer precision micromachining with negligible thermal damage, making them ideal for processing such sensitive materials. However, the formation and redeposition of oxide phases is a commonly observed side effect, especially when structuring in ambient air. This study investigates the effects of oxide formed after femtosecond laser treatment on self‐propagating properties, including propagation velocity and microstructure of Ni/Al reactive multilayer foils. Samples with varied line spacings between laser‐cut trenches are created. Scanning electron microscopy, high‐speed camera videography, and image analysis are employed to analyze the microstructure and quantify velocities. Thermal simulations enhance the understanding of the oxide's role in self‐propagating dynamics. The findings suggest that even a small oxide layer significantly decelerates the self‐propagating reaction. The oxide functions as a thermal ballast by absorbing the thermal energy generated during the reaction, without actively participating in the reaction mechanism.

Effective Carrier Lifetime in Ultrashort Pulse Laser Hyperdoped Silicon: Sulfur Concentration Dependence and Practical Limitations

Charge carrier lifetime is a crucial material parameter in optoelectronic devices and knowing the dominant recombination channels points the way for improvements. The effective carrier lifetime τeff of surface‐passivated hyperdoped (hSi) and nonhyperdoped “black” (bSi) silicon by quasi‐steady‐state photoconductance decay (QSSPC) measurements and its evolution upon controlled wet‐chemical etching are studied. Sample preparation involves the irradiation of Si by numerous ultrashort laser pulses either in SF6 for hSi or ambient atmosphere for bSi. Findings suggest that the hSi is composed of a double layer: 1) an amorphous resolidified top layer with about 92% of the total incorporated sulfur that accounts for the sub‐bandgap absorptance and 2) a crystalline layer underneath in which sulfur concentration tails off toward the Si substrate. The effective lifetime is deconstructed by a 1D simulation to quantify the impact of the local lifetime of the defect‐rich top layer, τtop. It is found that by the QSSPC method, a maximum τtop for 1) can be estimated. For 2), τtop between 2 and 8 ns is estimated. The bSi sample shows a faster lifetime recovery upon etching which suggests that in hSi samples purely laser‐induced defects are not limiting the carrier lifetime compared to sulfur‐related defects.

Transient optical properties change and dense plasma dynamics during water breakdown induced by ultrashort laser pulses

We propose a nonlinear transient coupling model to study the plasma dynamics during the interaction between ultrashort laser pulses and water. We conduct a quantitative characterization of the transient optical properties, energy deposition, and dense plasma dynamics in water with near-infrared ultrashort laser pulses by simulating the spatiotemporal coupling of the laser field with the plasma while calculating the time-varying relative permittivity. The results show that the free electron density in the dense plasma exceeds ≈1.0×1026 m−3, the plasma will move in the reverse direction of the laser pulse propagation, and the velocity is related to the incident laser intensity. An empirical model is established to quantify this reverse movement of dense plasma in the laser field taking into account the energy deposition. We also validate our coupling model by comparing the breakdown thresholds with experimental results and find excellent agreement.

Ultrashort pulse laser welding of alumina ceramic and titanium

The direct welding of alumina ceramic and titanium using ultrashort pulse laser was successfully achieved for the first time. Benefitting from the extremely high peak power of ultrashort pulse laser, ceramics can be melted at a very low laser power. The emerging welding method was conducted at room-temperature without the need for preheating. Within the joint, a combination of Ti and Al elements was observed. A newly formed TiO2 phase was identified at the Al2O3/welding seam interface. Under the optimal welding parameters (welding speed: 0.8mm/s, laser power: 19.52W), the joint achieved a maximum four-point bending strength of approximately 134.9MPa. This study proposed a new methodology for butt welding of ceramics and metals and shows a much higher efficiency than the commonly used ceramic-metal joining approaches such as brazing and diffusion bonding.

Reliable joining between MgAl2O4 and Ti6Al4V by ultrashort pulse laser

MgAl2O4 and Ti6Al4V, the greatly suitable materials for industrial application, are rarely explored to realize reliable bonding in current research. In this study, the transparent MgAl2O4 and Ti6Al4V were successfully bonded by innovative use of ultrashort pulse laser with ultra-high peak power. The high energy near the laser focus makes MgAl2O4 and Ti6Al4V mix and react with each other after absorbing the laser energy, thus realizing the bridge between them. The homogeneous mixing of all phases was observed in the joint and the newly generated Ti6O phase was identified at the interface. In addition, the effects of laser power and scanning speed on the microstructure and mechanical properties of the joint have been thoroughly clarified. This study creatively used femtosecond laser welding as a new mean to realize ceramic-metal joining, which shows great advantages in superior efficiency and precision compared with other welding methods.

Antibacterial property alterations induced by low zinc content in laser-structured brass

Brass, along with other copper-based alloys, exhibits advantageous antibacterial properties that can be further enhanced by altering the surface topography to increase bacterial adhesion. This enhancement is achievable through a higher contact area created by precise periodic structures, each approximately the size of a single bacterial cell. One method for generating these structures is ultrashort pulsed direct laser interference patterning (USP-DLIP). However, this process may induce chemical alterations in addition to topographical changes, depending on the substrate’s composition. To mitigate unfavorable chemical alterations, brass with a 15% zinc content was selected for this study. The antibacterial effectiveness of the modified surfaces was tested against Escherichia coli, providing initial insights into the interaction between bacteria and the substrate. The results indicate that modified brass with a 15% zinc content shows improved antibacterial activity. Overall, this research demonstrates that by modifying a surface with the appropriate chemical composition, effective bacterial elimination through contact can be achieved.

Reverse design of the ideal pulse for hollow capillary fiber post-compression schemes

The countless applications of ultrashort laser pulses in very different scientific areas explain the ongoing efforts to develop new strategies for the generation of light pulses with increasingly better characteristics. In this work, we theoretically study the application of the nonlinear reverse propagation method to produce few-cycle pulses with clean temporal profiles from standard post-compression setups based in hollow capillary fibers. By numerically solving the propagation of a desired goal pulse in the backward direction, we are able to predict the structure of the ideal input pulse that could be perfectly compressed in a given setup. Although the goal pulse cannot be chosen in a simple manner due to the fundamental symmetries of the nonlinear propagation equation, our analysis shows that the ideal pulse presents a recurring form and that, in general, both its intensity profile and phase must be shaped to recover the optimized goal output. Published by the American Physical Society 2024

Boosting radio frequency radiation with collisional processes in picosecond laser filamentation

When focused in air, ultrashort pulse lasers generate a plasma that produces ultrabroadband radio frequency (RF) radiation via both ponderomotive and plasma wake field current mechanisms. We have performed experiments with high energy pulses and pulse durations up to 5 ps, while holding the power constant. These longer pulses drive much higher electron densities and temperatures, especially as collisional processes become important, and we in turn have measured substantially increased RF generation. We have also developed a Drude numerical model of the ionization within the laser pulse, the heating due to collisions, and the ensuing current density evolution. We find that the low frequency scaling of the simulated current matches the experimental data, which indicates that the ponderomotive currents dominate the RF generation for these atmospheric pressure plasmas. However, the experimentally measured spectra also show an additional low frequency (1–10 GHz) component that grows with laser pulse length, which is consistent with the plasma wake surface wave RF also becoming important as the plasma temperature approaches 100 eV.

Influence of working parameters on multi-shot femtosecond laser surface ablation of lithium niobate

This paper presents an experimental investigation on the ultrashort pulsed laser ablation of 128° Y-cut Lithium Niobate (LiNbO3) using multi-shot fs-laser pulses at different pulse repetition frequencies (1, 10 and 100kHz). The ablation threshold fluence was observed to rapidly decrease as the number N of incident laser pulses increased, regardless of the repetition frequency. This behavior, compatible with the incubation effect, was accurately modeled by a power law. The calculated single pulse ablation threshold Fth,1=1.98±0.15J/cm2 is consistent with values reported in existing literature. The incubation coefficient S∗ appears to be independent of the repetition frequencies. In contrast, the asymptotic ablation threshold Fth,∞ decreased as the repetition frequency was increased. The study delves deeper into the impact of laser operational variables, including pulse energy, repetition frequency, total pulse count, and scanning speed, on the surface roughness and milled depth of fs-laser micromilled zones on LiNbO3 substrates. A discernible trade-off between achieving control over the obtained depth and the surface finish of the process was identified, providing valuable insights for achieving precise control over fs-laser processing of LiNbO3 surfaces.

Study on crystal to amorphous transition of nickel-based alloy processed by ultra-short pulsed laser shock peening

The main purpose was to investigate the process of crystal to amorphous transition in nickel-based alloy subjected to ultra-short pulsed laser shock peening. A molecular dynamics model of laser shock peened nanopolycrystalline nickel-based alloy was established using LAMMPS. The influence of shock velocity on the amorphous transformation in nickel-based alloy was analyzed. The results indicated that ultra-short pulsed laser shock peening with ultra-high strain rate plastic deformation promoted the increase and accumulation of defect for nickel-based alloy, contributing to the formation of amorphous structures.

Self-optimizing method and software for calibration and mapping of a laser system for laser machining

Short and ultrashort pulse lasers can ablate the vast majority of materials compared to other machining methods, making them the preferred tool in many niche applications. For the use in precision machining, such laser systems need to reach machining accuracy in the range of 1 µm. Usually, a galvanometric scanner is used in such laser systems to scan the laser beam across the scan field to mark or ablate the various materials. To achieve high accuracy, the overall laser system must be calibrated. However, such calibration is time-consuming and must be monitored at regular intervals and repeated if necessary. In this paper, a method and routine are introduced for self-optimizing calibration, monitoring and recalibration of a laser system. After calibration, the maximal deviation error of the laser system is 3.9 µm within the 30 mm x 20 mm scan field, and the optical axes are capable to match the positioning accuracy of the mechanical axes at 1.11 µm. A beam profiling camera is used to measure the beam position needed for calibration within the scan field. At the same, time other properties of the laser beam such as peak intensity, spot size and ellipticity are mapped within the scan field and is then used as a tool fingerprint of the laser beam across the scan field. Based on this, the influence of the position-dependent beam properties on the removal rate and surface quality is determined.

Influence of Laser Treatment of Ti6Al4V on the Behavior of Biological Cells.

This article explores the enhancement of material surface properties of Ti6Al4V, potentially applicable to dental implants, through ultra-short pulse laser systems. This study investigates potential connections between surface wettability and biocompatibility, addressing the challenge of improving variability in material properties with specific laser treatment. Several designed microstructures were manufactured using a picosecond laser system. After that, the wettability of these structures was measured using the sessile drop method. The basic behavior and growth activity of biological cells (MG-63 cell line) on treated surfaces were also analyzed. While the conducted tests did not conclusively establish correlations between wettability and biocompatibility, the results indicated that laser treatment of Ti6Al4V could effectively enlarge the active surface to better biological cell colonization and adhesion and provide a focused moving orientation. This outcome suggests the potential application of laser treatment in producing special dental implants to mitigate the issues during and following implantation.

Two-hole “desorption” mechanism of interstitial-vacancy pair generation visualized by avalanche-like color-center yield in synthetic diamond under ultrashort-pulse laser exposure

A model sectorial synthetic Ib-diamond plate with a minor concentration of substitutional atomic nitrogen C-centers was exposed in its bulk by tightly focused 0.2-ps, 525-nm laser pulses, coming at 80-MHz repetition rate and nJ-level variable pulse energies. Structural conversion into NV- and other carbon vacancy/interstitial-related centers was observed by infrared, optical and 3D confocal photoluminescence (PL) microspectroscopy, exhibiting in the optical range the ultra-broad (UV-near-IR) and smooth (structureless) absorption/photoluminescence spectra. PL intensity measured at the selected 480 and 530-nm wavelengths demonstrates its third power of the corresponding pulse energy dependences (∝E3) and avalanche-like dynamics as a function of laser exposure. Our analysis of electron-hole plasma photoexcitation and related generation of non-thermalized Auger carriers as a function of laser pulse energy (∝E1/2 and ∝E3/2, respectively) indicates that the known two-hole Auger-induced surface desorption mechanism could be anticipated in bulk diamond for interstitial-vacancy Frenkel pair generation, which rate could be evaluated via structural conversion rate for C-centers. At higher pulse energies tremendous interstitial-vacancy Frenkel pair generation rates could result not only in structural conversion of impurity-based color centers, but also in ultrafast “non-thermal” structural disordering in diamond.

Measurement and modeling of strain waves in germanium induced by ultrafast laser pulses

Transient reflectivity measurements are used to probe the strain waves induced by ultrashort laser pulses in bulk [100] germanium. The measurement signals are compared to purely analytical model functions based on the known material parameters for germanium. The modeling includes (i) a derivation of analytical solutions to the wave equation for strain waves coupled to the diffusion equation for heat and charge carriers and (ii) an expression for the impact on reflection coefficients that are caused by perturbations to the dielectric function but extended to cover a non-isotropic, uniaxial dielectric tensorial form. The model is held up against transient reflectivity measurements with an s- and a p-polarized probe and with a probe wavelength in the range of 502–710 nm. Excellent agreement is found when comparing the oscillatory shape of the measurement signals to the models. As for the magnitude of the oscillations, the models reproduce the overall trends of the experiment when using the previously published values for the elasto-optical tensor measured under static strain.

Similariton-like Pulse Evolution in an Er-Doped Fiber Laser with Hybrid Mode Locking

An Er-doped all-fiber ultrashort pulse laser with positive total net-cavity group-velocity dispersion is demonstrated based on a hybrid mode-locking mechanism ensured by single-walled carbon–boron–nitrogen nanotubes with coaction of the nonlinear polarization evolution effect. The generation regime with a similariton-like spectrum is obtained. The spectrum width is ~31.5 nm, and the minimal pulse duration is ~294 fs at full width at half maximum. The average output power is ~3.2 mW, corresponding to 0.376 nJ pulse energy and 1.25 kW peak power. The fundamental pulse repetition rate is ~8.5 MHz, with a signal-to-noise ratio of 60 dB. The standard deviation of average output optical power stability, measured for 12 h, is about ~1% RMS, and the maximum level of relative intensity noise (RIN) does not exceed <−120 dBc/Hz in the 30 Hz–1 MHz frequency range. To prove the similariton-like regime generation, we also studied numerically and experimentally the pulse evolution during propagation through a laser resonator and output single-mode fiber with anomalous dispersion.

Ultrafast processing of zirconia ceramics by transient and selective laser absorption

Ultrashort-pulse lasers (USPLs) are used in the machining of zirconia ceramics (ZrO2) because of their extremely high peak intensities. However, highly efficient and high precision microprocessing of ZrO2 remains challenging. In this study, we applied a transient and selective laser (TSL) processing technique by combining a USPL and a continuous-wave (CW) laser to realize ultrafast drilling of opaque ZrO2. Ultra-high-efficiency TSL drilling of ZrO2 was achieved with a processing efficiency 1800 times higher than that of USPL drilling. The material removal mechanism, hole formation process, and parameter dependence of TSL drilling were revealed by direct observation of the internal processing phenomenon on a timescale of microseconds. The variations in the processed surface characteristics, including surface morphology, elemental composition, and phase composition, are also presented. Subsequently, the effect of different excited electron regions occurring in the picosecond timescale induced by the USPL on the TSL drilling performance is discussed. Finally, the repeatability and stability of the TSL processing were verified by fabricating microarrays of different sizes. This study will contribute to revealing the ultrafast processing mechanisms of TSL processing in ZrO2 microfabrication. Furthermore, this technique can significantly broaden the applications of ZrO2 in the advanced electronic industry by greatly improving the microprocessing efficiency.

Single-step self-compression of ultrashort pulses below 20 fs by all-fiber supercontinuum generation

We present an ultrashort-pulsed laser source with time-widths as short as 20 fs at around 1.5 µm. For this purpose, the pulses have been spectrally broadened and temporarily compressed by means of supercontinuum generation in a highly nonlinear optical fiber. Inspired by numerical simulations, we have experimentally confirmed that the emission of a resonant dispersive wave is a key indicator for the optimum configuration to achieve maximum temporal compression. No dispersion control stage is needed, which results in the simplicity of the system. An extensive analysis has been made concerning the quality of the pulses obtained, particularly in terms of time-width (at half height and at 1/e height of the maximum) as well as their time-bandwidth product. The complex retrieval of the pulses shows that the smallest temporal width obtained experimentally is 17 fs. A comparison with values found in the literature also shows that our pulses are among the best that can be found with this technology.

Improving the fatigue property of diffusive film cooling holes in nickel-based single crystal superalloy via ultrashort pulse laser drilling coupled with abrasive flow machining

A novel hole-making technique, which combines ultrashort pulse (UP) laser drilling with abrasive flow machining (AFM), has been developed to enhance the surface quality and fatigue resistance of diffusive holes in nickel-based single crystal (NBSC) superalloys. This study conducted a comprehensive analysis of the surface morphology and metallurgical characteristics of the hole wall, evaluated by fatigue testing at elevated temperatures and fractography analysis. The findings demonstrate that AFM can effectively eliminate the solidified debris generated during UP laser drilling, significantly reducing surface roughness and inducing a rounded effect at the outlet acute zone of the diffusive hole. Such improvements have been shown to increase the fatigue life of the holes by up to 50.6 % compared to those without polish. Furthermore, the crystal plasticity finite element method (CPFEM) was employed to investigate the localized stress concentration and the accumulation of plastic slip around the diffusive hole, elucidating the mechanisms behind fatigue failure in NBSC superalloys. The study also discusses the influence of the different hole-making technology on the fatigue properties of diffusive holes, integrating CPFEM results with analyses of surface quality and fatigue fractography. The study conducted in this study can provide valuable guidance for the fabrication of diffusive holes in turbine blades. Furthermore, it can also improve our understanding of the fatigue failure mechanism of diffusive holes in NBSC superalloy.

Heat transfer in a thin metal film subjected to the ultra-short laser pulse modeled by a nonlinear two-temperature model

Heat transfer in a thin metal film subjected to the ultra-short laser pulse modeled by a nonlinear two-temperature model

Analytical solution to second-harmonic generation of ultrashort laser pulse

We present an analytical method to evaluate the second-harmonic generation (SHG) process of ultrashort laser pulses in a nonlinear medium within the small-signal regime. Building upon the broadband nonlinear coupled wave theory, we derive an analytical solution that allows one to investigate the fundamental physics and detailed dynamics of the nonlinear interaction between ultrashort laser pulses. We find that SHG for ultrashort laser pulses in a nonlinear crystal arises from a series of self-SHG and self sum-frequency generation processes involving different frequency components. We have applied the analytical method to investigate specific pulse profiles with Gaussian and sech functions and found good agreement between analytical theory and numerical simulations, validating the accuracy of the analytical approach. The analytical method enables one to explore the evolution of the second-harmonic wave under various pulse profiles, pulse widths, and crystal thicknesses, and offers a more efficient and insightful approach to study the nonlinear optical dynamics of ultrashort laser pulses.