Laser-metal Interaction Research Articles

Early-stage effects of residual charges in a metal target on emitted electrons induced by femtosecond laser–metal interactions

Electron emissions from a metal target surface may be induced due to the irradiation of the target by a femtosecond (fs) laser pulse. The emitted electrons will leave behind residual charges (which are positive) in the metal target near its surface. The residual charges may affect the evolution of the emitted electrons, which is called the “residual charge effect”. An intuitive belief could be that the residual charge effect is insignificant, because the huge number of free electrons in the interior region of the metal may quickly neutralize the residual charges. In this paper, the early-stage (at a time scale of less than ∼1 picosecond) residual charge effect has been investigated. The study shows that contrary to the above intuitive belief, the early-stage residual charge effect is very significant under the studied conditions, which has greatly slowed down the expansion of emitted electrons and enhanced their recombination back into the surface of the target. The study implies that to accurately study the early-stage fs laser-induced electron emission and other closely related processes, the residual charge effect should not be neglected.

Interference ablation of nanogratings with different duty cycles on gold films

Nanogratings with different duty cycles are directly written on gold films using interference ablation. After exposing the gold film to the interference pattern of ultraviolet laser beams, gold film in the bright fringes will be melted and ablated, whereas much weaker ablation occurs in the dark fringes. The duty cycle of gold nanogratings can be controlled by adjusting the pulse energy and the exposure times, which can be explained by the temperature rise and cumulative effect of the laser–metal interactions. These results introduce additional parameters for the fine fabrication of metal nanogratings based on interference ablation at a large scale and low cost.

Nanoscale modeling of conduction heat transfer in metals using the two-temperature model

The goal of this study is to investigate the energy transfer in metals due to electron–phonon (lattice) interaction using the two-temperature model (TTM). In TTM, molecular dynamics simulation and classical energy equation are used to find the lattice and electronic temperatures, respectively. An initial temperature profile is considered for the electronic temperature. Then, the electronic and lattice temperatures are determined until they reach equilibrium. This means that we excite electrons with assignment an initial temperature profile and simulate the subsequent energy relaxation process. To study the phase change during simulation, the radial coordinate numbers of atoms are calculated. The results show that the excited electrons may act as a heat bath and transport energy to other parts of the lattice. The same approach can be used to gain a high level understanding in very fast heat transfer phenomena especially in laser–metal interaction.

Numerical modeling and characterization of the laser–matter interaction during high-power continuous wave laser perforation of thin metal plates

The phenomena occurring during laser–metal interaction depend on a variety of parameters such as laser wavelength, intensity, and thermophysical and optical properties of the irradiated material. This work comprises a detailed study of the perforation process of thin iron and steel plates under CW laser radiation at a wavelength of 1.07 μm. Experiments were carried out over a wide range of intensities between 0.1 and 100 kW/cm2, and with beam radii in the millimeter and centimeter range. Additionally, we describe a method applying contact-free radiative temperature measurements, which allows the measurement of melt-through times without an exact knowledge of the plate's spectral surface emissivity. A detailed numerical model is presented and employed to analyze, interpret, and predict data gathered from the experiments. The model is shown to produce accurate predictions and is in good agreement with experimental data. Furthermore, our results indicate that in the investigated regime perforation is governed by gravitational and surface forces. Based on this hypothesis, a criterion for the onset of the perforation explaining the observed local minimum in the perforation time versus beam radius curve is presented.

Nanoscale heat transfer in direct nanopatterning into gold films by a nanosecond laser pulse

We investigate nanoscale heat transfer and heat-flux overlapping effects in nanopatterning through interactions between interferogram produced by 5-ns laser pulses at 355 nm and gold films. These mechanisms played different roles in direct writing of gold nanolines with different periods. Continuous gold nanolines were produced for large periods, where heat-flux overlapping is too small to effect the laser-metal interactions. Thus, the heat-transfer distance and direct laser-ablation determined the width of resultant gold nanolines. However, gold nanolines consisting of isolated gold nanoparticles were produced for small periods, where the overlapped heat-flux exceeds the threshold for removing or melting gold films.

The effect of emitted electrons during femtosecond laser–metal interactions: A physical explanation for coulomb explosion in metals

Recent experiments in the literature have observed Coulomb explosion (CE) in metals under femtosecond (fs) laser irradiation. This is different from the previous common belief that CE will be strongly inhibited in metals due to the existence of a large number of free electrons with good mobility and the associated screening effect. It is still not well understood why CE can occur in metals. CE requires a sufficiently high outwards pointing electric field in the metal near-surface region. Using a physics-based model, this study shows that during the early stage of fs laser–metal interactions, the emitted electrons due to fs laser irradiation are still very close to the metal target surface, whose effects also need to be considered. The emitted electrons will generate an additional outwards pointing electric field in the target near-surface region, and will also exert a repulsive force on the electrons flowing from the deeper region of the target towards its surface. These effects are helpful to the development of a large outwards pointing electric field in the target near-surface region. The model calculation considering the effects of emitted electrons shows that the electric field at around the target surface can exceed the CE threshold under the studied conditions. The study has provided a physical explanation for why CE can occur in metals under fs laser irradiation.

Analysis of melt-through process of 1.07 μm continuous wave high power laser irradiation on metal

Laser-metal interactions are influenced by various parameters, including laser wavelength and laser pulse duration. By proper adjustments of these parameters, one can create states that manifest different phenomena during laser ablation. In this work, we study laser melt-through of metal using 1 kW high power continuous laser with 1.07 μm wave length. A Ytterbium (Yb) doped fiber laser is used on the metal sample of varying thicknesses (0.1 ∼ 2 mm). In addition to providing measurements from the melt-through process, numerical study of thermal transport effect of laser heating and thermo-elastic response of metal are reported. The test-based simulation is shown to reproduce the thermal transport characteristics of beam-metal interaction at high power continuous wave irradiation with notable accuracy.

Laser pulses designed in time by adaptive hydrodynamic modeling for optimizing ultra-fast laser–metal interactions

Determining optimal temporal pulse shapes is an essential aspect for controlling the nature and the energetic characteristics of the ablation products following laser irradiation of materials on ultra-fast scales. In this respect, adaptive feedback loops based on temporal pulse manipulation have been inserted into a hydrodynamic code. The procedure enables us to reach the theoretical maximal temperature at a certain energy input. Several regimes have been considered with fluences ranging from the ablation threshold (F th=0.34 J/cm2) up to 10 J/cm2, proposing an optimal coupling for laser–solid and laser–plasma interactions in these fluence regimes. We determine shapes of optimal pulses on ultra-short and short scales (up to 42 ps) and forecast optimized interaction scenarios with fundamental control factors difficult to access experimentally. Simulations performed on aluminum reveal that ultra-short pulses are the natural better solution for localizing energy in space and time for F∼F th. For higher fluences, pulses spread over tens of picoseconds and ended by a final peak enable a better impulsive coupling with the nascent plasma, optimizing its maximal temperature.

Thrust generation through low-power laser-metal interaction for space propulsion applications

The micro-Newton thrust generation was observed through low-power continuous wave laser and aluminum foil interaction without any remarkable ablation of the target surface. To evaluate the thrust characteristics, a torsion balance thrust stand capable for the measurement of the thrust level down to micro-Newton ranges was developed. In the case of an aluminum foil target with 12.5 micrometer thickness, the maximum thrust level was 15 micro-Newtons when the laser power was 20W, or about 0.75μN/W. It was also found that the laser intensity, or laser power per unit area, irradiated on the target was significantly important on the control of the thrust even under the low-intensity level.

A two-dimensional comprehensive hydrodynamic model for femtosecond laser pulse interaction with metals

Femtosecond laser–metal interaction in air and the resultant early plasma evolution are investigated by a two-dimensional comprehensive hydrodynamic model in this paper. The model comprises a two-temperature model and a hydrodynamic model supplemented with a quotidian equation of state model, considering the relevant multiphysical phenomena during the laser–metal interaction. The experimental measurements for plasma expansion were carried out to validate the simulation results, using a shadowgraph technique and direct fluorescence measurement. The evolution of both the early plasma and plume plasma is investigated by the model. The early plasma is proved to be generated by electron emission and ambient gas ionization and splits into several portions during its expansion due to different mechanisms. The plume plasma comes from the target material ejection. The photoelectric emission is revealed to be the dominant electron emission mechanism at high laser intensities, while thermal emission is more important at low laser intensities. The electron emission process and early stage plasma are critical to ultrashort laser–metal interaction, especially at high laser intensities. Without considering this, the electron temperature can be overestimated by as much as 70%.

The Influence of Pulse Width and Energy on Temperature Field in Metal Irradiated by Ultrashort-Pulsed Laser

Abstract Based on the two-temperature coupling theory, a numerical model is established by using finite element method (FEM). Taking account of the spatial and temporal shape of the laser pulse and the temperature dependence of material properties, the heating process of femtosecond laser-metal interaction is presented. The temperature field of electron and lattice in metal is obtained. The influences of laser pulse width and energy on the electron and lattice temperature field and temperature gradient field are investigated. The numerical results indicate that the vertical lattice temperature gradient is about two orders larger than the horizontal lattice temperature gradient. Under the same irradiation pulse energy, the peak electron and peak lattice temperature at the center of laser irradiation decreases with the increase of laser pulse width.

Spectroscopic monitoring of penetration depth in CO 2 Nd:YAG and fiber laser welding processes

A method for on-line monitoring of the joint penetration depth is reported, based on the spectroscopic analysis of the optical emission collected from laser–metal interaction zone. This technique has been applied to quite diverse welding procedures (c.w. CO 2 laser, cw fiber laser and pulsed Nd:YAG laser) of stainless steel plates. Although the acquired spectra reveal different characteristics according to the employed laser source, a discrete contribution of several iron lines can be highlighted in both types of investigated welding processes. Starting from the measurement of the intensities of those lines it is possible to calculate the plasma electron temperature, which was found to decrease as far as the laser power is enhanced together with the penetration depth, in static as well as dynamic conditions. Such a behavior does not correspond to a real decrease of the plasma plume temperature but has to be ascribed to the position of the optical collimator collecting light only from the top of the keyhole. Indeed, for deeper penetrations the hottest core of the plume moves down into the vaporized capillary so that the plasma temperature measured on top of the keyhole appears lower at higher incident powers. The electron temperature signal could be useful for the development of a feedback loop system of the laser power to control the weld penetration depth.

Temperature-dependent absorptance of painted aluminum, stainless steel 304, and titanium for 1.07 μm and 10.6 μm laser beams

We measured the temperature-dependent absorptance of metals (Al, Ti, SS304) for continuous beams from 1.07 μm fiber laser and 10.6 μm CO 2 laser using power sensors and infrared (IR) pyrometers. The absorptance measurements were repeated for metals with three different paint coatings. For measurements at elevated temperatures up to the melting point, integrating sphere is not practical since high temperature radiation from a heated target disturbs weak output from the sphere considerably. Our results provide how each metal, whether coated or uncoated, absorbs the infrared beams as temperature is elevated to a melting point. A polynomial approximation to the measured absorptance of each target is provided for modeling of the laser–metal interaction at elevated temperatures.

Direct patterning in sub-surface of stainless steel using laser pulses

This paper reports for the first time on the direct creating microcavities in sub-surface of stainless steel using a single Nd:YAG laser pulse. The low peak power density is used in the process, which is in the order of 1 MW/cm(2). The formation of the microcavities in the sub-surface of stainless steel is an evidence of volume expulsion during laser-metal interaction. Direct patterning in the sub-surface of stainless steel is demonstrated by realizing a series of microcavities to form a pre-designed pattern. Potential applications of sub-surface patterning in metal, such as security marking, micro-heater, micro-insulator and micro-sensor, are discussed.

Thermomechanical wave propagation in gold films induced by ultrashort laser pulses

The thermomechanical behavior of gold films caused by ultrashort pulse lasers is investigated using a combined continuum-atomistic approach with the inclusion of the hot electron blast force. During the very early laser-metal interaction, a nonequilibrium thermal state and a significant hot electron blast wave are generated in the films. The laser heating leads to an initial compressive stress with the peak occurring near the irradiated side of the films. For a 100-nm film, a conversion from compression to tension starts in the mid-portion, the resulting tensile stress converts back to compression, and then the above stress conversions repeat. For the thicker films of 500 nm and 1 μm, the tensile stress starts from the irradiated side due to the free surface stress reflection, and the resulting two-fold shock wave comprising tension and compression propagates towards the rear surface and then reflects back to the front-side. The effect that the thermal stress decreases with increase of the pulse duration is more pronounced in thinner films. Based on the simulated dynamic stress responses, nonthermal damage to the 100-nm film could occur in its mid-depth region while the material could be nonthermally removed from either the front or rear-side of the 500-nm and 1-μm films.

Numerical Analysis on the Feasibility of Laser Microwelding of Metals by Femtosecond Laser Pulses Using ABAQUS

Ultrafast lasers of subpicosecond pulse duration have the potential for laser microwelding of micronscale fusion zone. Due to the extremely short pulse duration, laser-metal interaction involving ultrafast laser pulses should be analyzed using the two-temperature model. In this study, the two-temperature model is analyzed using ABAQUS to study the feasibility of laser microwelding with ultrafast laser. A material model is constructed using material properties and the subsurface boiling model. The model is validated using experimental results from the literature. Laser processing parameters of repetition rate, pulse duration, and focal radius are then investigated, in terms of molten pool generated in the material and requirements on those parameters for laser microwelding using ultrafast lasers are discussed.

High-intensity nanosecond-pulsed laser-induced plasma in air, water, and vacuum: A comparative study of the early-stage evolution using a physics-based predictive model

A comparative study has been performed for properties (temperature, density, and electron Coulomb coupling constant) of plasma induced by high-intensity (∼GW∕cm2) nanosecond laser-metal interactions in air, water, and vacuum. The study is for early-stage (t≲30ns) plasma evolution, where the above plasma properties are very difficult to measure experimentally and hence a comparative property study has been rarely reported in literature. In this paper a physics-based predictive model is used as the investigation tool. The model was verified based on experimental measurements for the early-stage plasma pressure and front propagation and the late-stage (t≳30ns) plasma temperature and electron number density, which are relatively easy to measure. Therefore, the experimentally verified model can provide reasonably accurate information on the difficult-to-measure plasma temperature and density in the early-stage at least in the semiquantitative sense, and the information will be very useful for the fundamental laser plasma study and relevant laser applications. It has been found that plasma with very different temperatures and densities can be created in different media.

Langmuir probe investigation of surface contamination effects on metals during femtosecond laser ablation

Characterisation of the plasma plume induced by femtosecond laser–metal interactions has been carried out using a Langmuir probe. A double peak distribution of ablated ions and electrons has been recorded during time of flight (TOF) experiments for three metals studied (Ag, Cu and Ni). The first peak which occurs earliest in time is attributed to a surface layer of contaminants on the metal surface as it is shown to disappear after several laser shots. The re-growth of this peak, thought to be due to a recontamination process on the surface of the metal, is the subject of this paper. Two re-contamination mechanisms were considered; adsorption of contaminants from the ambient gas, and surface diffusion effects from the surrounding contaminants. Re-contamination rates for Ag, Cu and Ni were studied under two distinct gas pressures to investigate the contamination effects from the ambient. Effects arising from surface diffusion were investigated by raising the temperature of the metal sample to increase the surface mobility of the contaminants. The total contribution of contamination species present in the ablation plume was estimated by conducting angular distribution measurements of the plume. Surface diffusion of the surrounding contaminants was found to be the dominant recontamination process.

Thermal affected zone obtained in machining steel XC42 by high-power continuous CO 2 laser

A high-power continuous CO 2 laser (4 kW) can provide energy capable of causing melting or even, with a special treatment of the surface, vaporization of an XC42-steel sample. The laser–metal interaction causes an energetic machining mechanism, which takes place according to the assumption that the melting front precedes the laser beam, such that the laser beam interacts with a preheated surface whose temperature is near the melting point. The proposed model, obtained from the energy balance during the interaction time, concerns the case of machining with an inert gas jet and permits the calculation of the characteristic parameters of the groove according to the characteristic laser parameters (absorbed laser energy and impact diameter of the laser beam) and allows the estimation of the quantity of the energy causing the thermal affected zone (TAZ). This energy is equivalent to the heat quantity that must be injected in the heat propagation equation. In the case of a semi-infinite medium with fusion temperature at the surface, the resolution of the heat propagation equation gives access to the width of the TAZ.

Spectroscopic diagnostics of plasma during laser processing of aluminium

The role of the plasma in laser–metal interaction is of considerable interest due to its influence in the energy transfer mechanism in industrial laser materials processing. A 10 kW CO2 laser was used to study its interaction with aluminium under an argon environment. The objective was to determine the absorption and refraction of the laser beam through the plasma during the processing of aluminium. Laser processing of aluminium is becoming an important topic for many industries, including the automobile industry. The spectroscopic relative line to continuum method was used to determine the electron temperature distribution within the plasma by investigating the 4158 Å Ar I line emission and the continuum adjacent to it. The plasmas are induced in 1.0 atm pure Ar environment over a translating Al target, using f/7 and 10 kW CO2 laser. Spectroscopic data indicated that the plasma composition and behaviour were Ar-dominated. Experimental results indicated the plasma core temperature to be 14 000–15 300 K over the incident range of laser powers investigated from 5 to 7 kW. It was found that 7.5–29% of the incident laser power was absorbed by the plasma. Cross-section analysis of the melt pools from the Al samples revealed the absence of any key-hole formation and confirmed that the energy transfer mechanism in the targets was conduction dominated for the reported range of experimental data.