Ablation Threshold Fluence Research Articles

Investigation of ablation threshold and microchannel fabrication on stainless steel using ultrafast laser

ABSTRACT Ultrafast laser processing is a highly versatile tool for creating microscale structures in many materials. The ablation and micromachining characteristics of AISI 304 stainless steel were examined using a femtosecond-pulsed laser. Parameters such as power, scanning speed, transverse overlap, scanning strategies, and shielding gases were systematically varied. The ablation threshold fluence for SS 304 was calculated as 0.16 J/cm2. Two ablation regimes were observed: the gentle regime, where smooth crater walls were formed under low pulse energy (below 20 µJ), and the strong regime, characterized by rough walls created under high pulse energy (above 20 µJ). The optimal parameters for fabricating microchannels on SS 304 surfaces comprised a laser power of 100 mW, a scanning speed of 30 mm/s, a line-to-line distance of 5 µm, and a parallel scan strategy along the major axis. This study provides a practical approach for enhancing the microchannel fabrication process on stainless steel.

Amorphization and Ablation of Crystalline Silicon Using Ultrafast Lasers: Dependencies on the Pulse Duration and Irradiation Wavelength

AbstractUsing lasers to achieve controlled crystallographic phase changes in silicon with high spatial precision promises new manufacturing solutions in semiconductor technologies, including silicon photonics. Recent demonstrations of improved amorphization thicknesses position ultrafast lasers as an optimum tool to meet current challenges. Here, the literature on silicon transformations is reviewed and complemented with new experimental data. This includes amorphization and ablation response as a function of pulse duration (τ = 13.9 to 134 fs at λ = 800 nm) and laser wavelength (λ = 258 to 4000 nm with τ = 200 fs pulses). For pulse duration‐dependent studies on Si(111), the amorphization fluence threshold decreases with shorter durations, emphasizing the significance of non‐linear absorption in the range of considered conditions. For wavelength‐dependent studies, the amorphization threshold increases sharply from λ = 258 to 1030 nm, followed by near‐constant behavior up to λ = 3000 nm. Conversely, the ablation threshold fluence increases in these specified ranges. Differences in the obtained amorphization thicknesses on Si(111) and Si(100) are also discussed, identifying an anomalously large fluence range for amorphization at λ = 258 nm. Finally, the question of the lateral resolution, shown as independent of the interaction nonlinearity is addressed.

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.

Wavelength selection for femtosecond laser processing of materials: Comparison of ablation efficiency and surface quality

We investigated the effects of the femtosecond laser wavelengths λ = 1035, 515, and 345 nm on the laser ablation of 64FeNi alloy (Invar). The ablation rate and ablation efficiency for the three wavelengths were obtained through material-removal experiments of stationary irradiation and laser scanning, respectively. Among laser–matter interaction parameters, the effective penetration depth (α−1) and ablation threshold fluence (Fth) could be estimated from the experiments and were found to be proportional to λ. The ablation rate and ablation efficiency for the peak fluence range of 0.2 J/cm2 < F0 < 1.0 J/cm2 and F0 > 1.0 J/cm2 were the highest at λ = 515 and 1035 nm, respectively, which could be confirmed by using analytical models with α−1 and Fth. The smoothest (roughest) processed surfaces could be obtained at λ = 345 nm (1035 nm) owing to the shortest (longest) periodicity of laser-induced periodic surface structures. Thus, surface smoothing at λ = 345 nm over a certain ablation depth after ablation at λ = 1035 nm, i.e., double-wavelength ablation, can be used for improving surface quality. These results can help in the selection of the proper λ for implementing precise laser micromachining with high productivity.

Indium tin oxide ultrafast laser lift-off ablation mechanisms and damage minimization.

We draw comparisons between the ablation and damage mechanisms that occur for both film and substrate irradiation using atomic force microscopy, scanning electron microscopy, and pump-probe reflectometry. For substrate irradiation, energy absorbed at the film-substrate interface creates a confined energy situation, resulting in a photomechanical lift-off. A partial ablation at the edges of the ablated zone formed the burr and was reduced in height by minimizing the area subject to the partial ablation threshold fluence. Substrate damage is understood to arise from free electron diffusion from indium tin oxide and subsequent laser heating. We establish a process window for substrate irradiation in a single-pulse ablation regime between approximately two to three times the ablation threshold of 0.18 J/cm2, validating the process window seen in literature and provide a crucial understanding for the ablation mechanisms of transparent conductive films.

Ablation characteristics of pre-heated region in laser line scanning process

The conditions for obtaining a completely connected ablated line on a Ag nanoparticle (NP) layer were investigated. In the scanning ablation process, the area pre-heated by a previous laser pulse is sintered, and its physical properties are changed. We examined the line patterning of AgNP layers at different sintering temperatures. The ablation thresholds of AgNP layers that were dried, sintered at 150 °C, and sintered at 200 °C were 0.211, 0.536, and 1.84 J/cm2, respectively. Some of the line patterns in the AgNP layers were not completely ablated when the applied laser fluence was higher than the ablation threshold fluence for each AgNP layer. A higher fluence was needed to form an ablated line properly. The line ablation threshold was approximately two to four times the ablation threshold. The ablated linewidth at the line ablation threshold was similar for all sintering temperatures.

Investigation of Ablation Efficiency of Stainless Steel Using Pulsed Lasers in Burst Mode

Due to its high corrosion resistance and mechanical properties, stainless steel is commonly used in various industrial applications. Although different types of stainless steel are similar in their chemical composition, they can differ significantly in their thermal diffusivity. This property is relevant in the ability of a material to conduct heat and thus, in laser processing. In this frame, this study compares the ablation efficiency and characteristics of polished stainless steel samples of the alloys AISI 304, AISI 420, and AISI 316Ti. They are irradiated with single ultrashort pulses having pulse durations between 250 fs and 10 ps as well as using GHz burst modii. The goal is to investigate the differences in both the ablation threshold and the ablation rate to improve the ablation efficiency. The results show that shorter pulse durations lead to a more efficient ablation process. On the other hand, GHz bursts are found to be, in general, less efficient. In addition, there is a significant difference in the surface morphology depending on the process parameters. The differences in the thermal diffusivity do not significantly influence the ablation threshold fluence but surface morphology and the ablation rate.

Spot size dependence of the ablation threshold of BK7 optical glass processed by single femtosecond pulses

A comparative study of the evolution of the diameter and depth of the ablated craters, while varying systematically both pulse energy and spot size is reported. It reveals that the surface ablation threshold fluences double-checked by the diameter-regression method and a multiphoton-absorption-based fit show a clear dependence on the radius of the processing laser beam on the BK7 optical glass surface. Tighter focusing results in a rise from 6.5 J/cm2 up to nearly 11 J/cm2 within the 26.1–10.1 µm radius domain. The evolution of the time-integrated energy of the reflected part of every single pulse with rising pulse energy indicates three types of the optical response of the material, depending on the actual energy of the processing pulse. At the threshold where the first and second section join, which shifts to lower pulse energies with decreasing spot size, the initial reflectivity, assigned to the permanent reflectivity of the BK7 glass, suddenly increases. This rise indicates plasma mirror formation on the target surface. The dependence of the reflectivity enhancement and the ablation thresholds on spot size is very similar. The values remain equal for spot radii at and above 13.5 µm, followed by a rise with decreasing spot size. This reveals that the plasma mirror formation could be affirmed as the dominant process behind the spot size dependence of the ablation thresholds in our case. The appearance of double-crater structures is consistent with the evolution of air ionization, resulting in lower reflectivity at the highest energies. This assignment is further corroborated by proving that the surface defect model could hardly be accounted for the results. The findings support the consideration of the spot size as a control parameter.

Subthreshold Laser Ablation Measurements by Langmuir Probe Method for ns Irradiation of HfO2 and ZrO2.

The unbiased Langmuir probe (LP) method was used to perform measurements on HfO2 and ZrO2 samples around the laser ablation threshold on a wide range of irradiation conditions. Important changes in the lifetime (from ms to μs) and the shape of the charge particle current were seen with the increase of the laser fluence. The ablation threshold was estimated by evaluating the overall average ablated charge as a function of the laser fluence. Above the ablation threshold, the generation of high kinetic species is seen, which can reach several keV. An important jump in ion acceleration potential is observed for values above 1 J/cm2, which coincides with the dominant presence of negative ions in the plasma. The evolution of several plasma parameters (ion density, expansion velocity, electron temperature, Debye length) was investigated and correlated with the fundamental ablation mechanism involved in various irradiation regimes. The LP data were correlated with COMSOL simulations on the maximum surface temperature reached during irradiation. Important correlations between the evaporation and melting processes and ablation threshold fluence and ion acceleration phenomena are also reported.

Validation of ablation model for polyethylene using pulsed x-ray and proton exposures

The surface erosion of polyethylene is interrogated using pulsed x rays at the Z Machine (Sandia National Laboratories) and with proton beams at the Gamble II generator (Naval Research Laboratory) to validate a coupled model for volumetric thermal ablation, photoionization, finite-rate decomposition, and molecular recombination of radicals. The intense radiation pulses (up to ∼1014W/m2 over tens of nanoseconds) are used to generate one-dimensional vapor flows with low ionization fractions and a simplified geometry compared to typical laser ablation, allowing for evaluation of the model under local thermal equilibrium conditions. Areal momentum carried by the ensuing uniaxial hydrodynamic shock is used to indicate the extent of ablation. The threshold fluence for ablation is found to be in close correspondence with the bulk melt transition, and reasonable agreement with the model is obtained for peak temperatures in polyethylene up to 5500 K and heating rates up to 1011K/s where thermal decomposition reactions are also active.

Optical and Thermal Behavior of Germanium Thin Films under Femtosecond Laser Irradiation.

In this study, we theoretically investigate the response of a germanium thin film under femtosecond pulsed laser irradiation. Electron and lattice temperatures, as well as material-specific optical properties such as dielectric function and reflectivity, were calculated during the irradiation using an extended two-temperature model coupled with the carrier density rate equation and the Drude model. Melting and ablation fluence thresholds were also predicted, resulting in 0.14 J cm-2 and 0.35 J cm-2, respectively. An ultrafast change in both optical and thermal properties was detected upon laser irradiation. Results also indicate that thermal melting occurs after germanium takes on a metallic character during irradiation, and that the impact ionization process may have a critical role in the laser-induced thermal effect. Therefore, we suggest that the origin of the thermal modification of germanium surface under femtosecond laser irradiation is mostly due the impact ionization process and that its effect becomes more important when increasing the laser fluence.

Ultrafast laser ablation of copper with GHz bursts: A numerical study on the effects of repetition rate, pulse number and burst fluence on ablation efficiency and heat-affected zone

Ultrafast laser burst ablation (ULBA) with high repetition rates up to GHz has emerged as a promising method for improving ablation efficiency and reducing heat-affected zone (HAZ). However, the effects of different laser parameters are not well understood. This article presents a numerical study by an improved two-temperature model on the influences of repetition rate, pulse number and burst fluence on ablation efficiency and HAZ in ULBA of copper. Due to thermal accumulation, increasing the repetition rate enhances the maximum ablation efficiency, by over 150% at 3456 MHz. Interestingly, large pulse numbers also assist this effect by extending the period of thermal accumulation. However, if the burst fluence is too low, increased pulse numbers will reduce the ablation efficiency due to insufficient pulse fluences. Burst duration and fluence are determined to have the greatest impact on HAZ thickness. To minimize HAZ, the burst duration needs to be short, by raising repetition rates or reducing pulse numbers, to decrease the thermal diffusion time. The dependences of ablation optimum and threshold fluences on pulse numbers and repetition rates are analyzed and summarized in simple formulae. This study unveils the impacts of laser parameters in ULBA and provides guidelines for ablation efficiency and HAZ optimization.

Laser ablation on coated metal gravures for roll-to-roll printed electronics

Ultrafast laser micromachining plays an important role in the manufacturing of electronics. Especially, rotogravure processing produces high-resolution patterns for roll-to-roll printed electronic circuits. However, this technique still poses some limitations. This study introduces a new technique for coating gravures on a “soft” metal to improve the precision of rotogravure processing for manufacturing roll-to-roll electronics. The coating is termed “soft” because it has a lower ablation threshold fluence than the metal gravure core. The resulting U-shaped grooves offer several advantages over the V-shaped grooves typically formed through laser ablation on a single-layer material. Conductive ink transfers more completely from the U-shaped grooves, offering increased precision with respect to the resulting parts and saving ink.

Electron dynamics in fused silica after strong field laser excitation detected by spectroscopic imaging pump-probe ellipsometry

Laser ablation of dielectrics by ultrashort pulsed laser radiation is induced by nonlinear photoexcitation, such as multiphoton and tunneling processes, and the subsequent impact excitation, due to electron-electron collisions. Previous theoretical and experimental studies demonstrate that these two phenomena strongly depend on the transient optical properties of the dielectric and the electron-electron collision times. To estimate these transient optical properties and the collision time, the excited electrons are usually considered as a nearly free-electron gas, being described by the Drude theory. All previous experimental studies support using the Drude theory by time-resolved measurements of the transient reflectance during and after irradiation. However, a comprehensive characterization of the transient complex refractive index, and, thus, a more complex view of the optical properties upon irradiation above the ablation threshold fluence was still missing. Therefore, this paper presents the time-, spectral-, and fluence-resolved complex refractive index of fused silica during the interaction with ultrashort pulsed laser radiation (800 nm, 40 fs, and 4.4 J/${\mathrm{cm}}^{2}$) measured by spectroscopic imaging pump-probe ellipsometry. Comparing the state-of-the-art Drude model to the experimental data reveals that, although the Drude model replicates the transient reflectance of the material, it deviates significantly from the transient refractive index in the considered temporal range up to 200 fs after irradiation. Thus, the Drude model cannot correctly describe the transient optical properties, which leads to the assumption that the electrons cannot be considered as a nearly free-electron gas at this time. Alternatively, a Lorentz model, considering the excited electrons as bound, can replicate the measured data much better, which indicates that the electrons might be excited into localized states.

The role of fluence in determining the response of thin molybdenum films to ultrashort laser irradiation; from laser-induced crystallization to ablation via photomechanical ablation and nanostructure formation

The selective processing of Mo at low temperatures is challenging, especially in advanced manufacturing on flexible and heat-sensitive substrate due to its higher melting temperature. The key role of fluence in determining the response of thin Mo films to ultrashort laser irradiation is considered in this study. At low fluences, the electrical properties of Mo are enhanced by a localized laser-induced crystallization mechanism; the electrical mobility of Mo is increased and the contact resistance between Mo-Si interface is reduced. At higher fluences, selective patterning of Mo proceeds without impacting the Si layer and the threshold fluence for ablation increases with the film thickness of Mo. Two fluence dependent ablation mechanisms are observed depending on the Mo film thickness. For thin films of thicknesses 20 nm and 40 nm, selective ablation proceeds only by a photothermal interaction. For 60 nm and 80 nm thick films, selective ablation proceeds by both photomechanical and photothermal interactions at two-separate higher fluence regimes, respectively. Interestingly, between these two ablation regimes, a non-ablative nanostructuring regime occurs. The study provides a concise overview of the process window for implementing the laser-induced modifications to Mo layers with minimal impact to the substrate using single wavelength ultrashort pulse laser.

A quantitative method for in situ pump-beam metrology in 4D ultrafast electron microscopy

We report a method for measuring spot size and focusing conditions of the femtosecond (fs) excitation laser in situ at the specimen location in 4D ultrafast electron microscopy (UEM). The method makes use of threshold laser ablation behaviors of thin amorphous carbon membranes. For Gaussian beam profiles and for ablation threshold fluence values, we analytically derive expressions describing the relationship between ablated hole size and the actual laser spot size. Using these expressions, we developed experimental procedures for characterizing the shape and spot size of the pump beam at the specimen. We demonstrate the viability of the approach for incident excitation wavelengths of 343 nm and 515 nm, thus illustrating the applicability of the method to a range of optical wavelengths without modification. Further, we show that by measuring ablated hole size as a function of focusing condition, a full metrological characterization of the Gaussian beam propagation properties can be performed. Finally, we find good agreement for spot sizes determined with this method and with those determined by extrapolation from measurements taken outside the microscope column. Overall, this method is a simple, cost-effective means for accurate and precise determination of key pump-beam parameters in situ at the specimen location in UEM experiments.

Picosecond pulsed laser machining of hardened martensitic stainless steel drive shaft

A fundamental method for estimating the laser parameters and for predicting the achievable ablation rate is demonstrated for hardened martensitic stainless steel X46Cr13. The laser ablation rate is evaluated for a range of pulse energies. The effective threshold fluence for ablation and the effective penetration depth are determined for a low and a high energy regime applying the Neuenschwander model. These parameters are used for numerical prediction of the ablated geometry. A very time efficient processing strategy and parameter set is selected and applied for laser machining of micro channels into a high precision drive shaft to increase friction for torque transmission leading to a processing time of 30 s per part, which corresponds to a structuring rate of 0.88 mms-1 . The deviation between the simulation and the ablated depth in the experiment are measured to be below 7 % at a total ablation depth of 25 µm. This demonstrates that the Neuenschwander model is suitable to determine the parameters needed for the prediction of the geometry resulting from an ablation process on martensitic stainless steel.

Ultra-short pulsed laser powder bed fusion of Al-Si alloys: Impact of pulse duration and energy in comparison to continuous wave excitation

Laser assisted powder bed fusion of Al-40 wt%Si using ultra-short laser pulses is presented. The effects of the pulse duration as well as the pulse energy on the melt pool size, solidification morphology and material properties have been investigated. The experiments have been carried out with a mode-locked fiber laser system delivering pulses from 500 fs up to 800 ps at a wavelength of 1030 nm. Comparative investigations have been performed using a continuous wave Yb-fiber laser operating at 1070 nm. The results show that the melt pool width is reduced at shorter pulse durations and higher repetition rates while maintaining the same average power. Additionally, keyhole melting is achieved in pulsed operation after exceeding the threshold fluence for single pulse ablation. In comparison to continuous wave radiation, powder bed fusion using ultra-short laser pulses leads to a more uniform melt pool shape with refined primary Si and eutectic structure that is accompanied with an improvement of the mechanical properties.

Unravelling ultrashort laser excitation of nickel at 800 nm wavelength

The optical response of nickel is studied in a wide range of laser fluence, below and above the ablation threshold, by self-reflectivity measurements of ultrashort 800 nm single laser pulses. At the ablation threshold, the reflectivity remains unchanged with respect to its unperturbed value irrespective of the pulse duration, from 15 to 100 fs, consistently with the steadiness of the laser-induced ablation threshold fluence F th for all pulse durations tested. Until the ablation threshold (F ⩽ F th) and whatever the pulse duration, the disturbances caused to the initial structure of the electron gas distribution by the laser energy deposition are limited, having no significant impact on the transient optical response of nickel and on its ablation threshold. At higher laser fluences (F > F th), the reflectivity becomes rapidly dominated by the contribution to the optical response of the fast-thermalized free electrons (4s-band) with energy largely above the Fermi energy level. In these conditions, the reflectivity decreases for all pulse durations enhancing laser energy coupling and larger optical absorption at the surface of nickel. The optical response of nickel under ultrashort (15–100 fs) irradiation is thus fully elucidated on a wide range of fluence (0.3 F th–30 F th) and for pulse duration down to few-optical-cycle pulse duration. As a key parameter for benchmarking laser-matter interaction in poorly known conditions yet, the evolution of the effective electron collision rate is determined as a function of fluence and pulse duration in very good consistency with experiments.

Heat accumulation and surface roughness evolution in CO2 nanosecond laser ablation of quartz glass

Quartz glass is frequently used for optical components, but traditional machining methods, particularly for aspherical optics, are often associated with long process times and high costs. One possible approach to achieve higher efficiencies and higher speeds in contactless processing is laser ablation using CO2 laser radiation. This study investigates the evolution of surface topography during laser ablation of quartz glass using nanosecond pulses from a Q-switched CO2 laser beam source. The special focus of a broad empirical study lay on ablation depth, ablation rate, ablation efficiency and resulting surface roughness for an ablation process using 300 ns laser pulses at a maximum laser power of 115 W and pulse frequencies up to 150 kHz. Ablation efficiency shows a characteristic logarithmic dependency on laser fluence. It was revealed that local heat accumulation significantly affects the optical penetration depth, which reduces the threshold fluence for multi-pulse laser ablation down to approx. 0.37 J/cm2. Ablation rates of up to 1.48 mm3 min−1 W−1 were achieved, which exceeds results achieved for laser ablation using cw laser radiation or ultra-short pulses. An ablation rate of up to 170 mm3/min is a particularly noteworthy result. Therein, a maximum ablation efficiency of approx. 64% shows that the largest part of the incident laser energy was used for material ablation. Finally, based on a simplified model, numerical calculations and experimental results, pulse stability was identified to have a decisive impact on the evolution of the resulting surface roughness. The resulting surface roughness Sa is typically increased in pulsed laser processing and depends linearly on standard deviation of ablation depth and the square root of the number of ablation layers.