Short-duration Laser Pulses Research Articles

Research on the residual stress induced by square-spot laser shock peening on 2024-T351 specimens

Laser peen forming (LPF) is an appealing technique for forming metal sheets using high-energy, short-duration laser pulses. The deformation of the target metal plate is closely related to the magnitude and distribution of laser-induced residual stress. Consequently, the relationship between process parameters and residual stress is worth researching. In this research, two process parameters in LPF, laser energy and coverage ratio (spot distance essentially), and one workpiece parameter, plate thickness, were examined through an element method (FEM) of multiple square-spot laser shock peening (SSLSP). Corresponding experiments of SSLSP on aluminum alloy 2024-T351 test blocks were conducted, together with an X-ray diffraction (XRD) residual stress measurement and a surface morphology observation. The FEM simulation and experimental results show that congested laser spots had a significant influence on the magnitude of compressive residual stress; higher laser energy was beneficial to the depth of the compressive stress layer but could decrease its magnitude. Therefore, for better forming ability, higher laser energy and a higher coverage ratio are beneficial; for surface strengthening, laser energy should not be too large, and the coverage ratio should be larger than 100% to ensure that the residual stress on the treated surface is compressive, resulting in better surface integrity.

Minimum separation between two pump pulses for ultrafast double magnetization switching in GdFeCo

Femtosecond laser ultrafast thermally induced magnetization switching (TIMS) has also attracted much attention due to its ability to trigger a single switching at the picosecond timescale. Current studies have shown that after a TIMS excited by a laser pulse, excitation of the switch again via TIMS does not require equilibrium between the subsystems. In this work, the main investigation is on the various possible cases of magnetization dynamics in GdFeCo under two short-delayed pulse excitations, as well as the factors limiting the minimum separation for double TIMS. These conditions are relevant for the potential application of TIMS to memory devices as it affects both the speed limit at which rewritten data is available and demonstrates the importance of spatial confinement of a laser pulse to bit size. The results show that low energy and short pulse duration lasers are prerequisites for double TIMS in GdFeCo based on simulations of atomic spin dynamics. By changing the damping constants of the alloy, we can shorten the minimum pulse separation between two pump pulses for double TIMS to 2 ps to approach terahertz frequency of write/erase cycles.

Damage effects of high-intensity laser pulse on W for fusion applications: modelling and experiments

In future nuclear fusion reactors plasma-facing materials (PFMs) will be exposed to transient high energy events such as disruptions, edge localized modes (ELMs) and vertical displacement events (VDEs) which may cause melting and vaporization leading to component damage and plasma contamination. The interaction between plasma and refractory metals, namely bulk tungsten (W) and W deposited by plasma spraying (PS-W), has been simulated through a single laser pulse of high energy (≈ 4 J) and short duration (≈ 15 ns). The morphology of the zone affected by laser pulse has been then examined by scanning electron microscopy (SEM) and 3D optical surface profilometry. Both an analytical and a numerical thermal model of the phenomenon based on the specific conditions of heat transfer from the laser spot into each irradiated material were developed. The attention has been focused on the erosion occurring on the material surface because this is the primary cause of damage of PFMs and plasma contamination. Numerical, analytical, and experimental results were found in good agreement.

Remote laser spot welding of AISI 430 sheets by fiber lasers—A phenomenal effect in refining weld microstructure with nanosecond pulses

In the present work, we have compared the results of microstructural and mechanical property characterization of AISI 430 ferritic stainless steel spot welds made remotely using a repetitive nanosecond pulsed fiber laser and a CW fiber laser. Optical/electron microscopy, microhardness test, and tensile shear tests were performed for evaluation of the welds. Welds made by the pulsed nanosecond laser were found to be superior due to the presence of the very narrow heat-affected zone along with finer grains in the fusion zone. The welds were also found to withstand more load before fracture and were more ductile. Use of short duration laser pulses with lower heat input were found to be responsible for the refinement of grains in the fusion zone and improvement of mechanical properties of nanosecond laser weld specimens.

Intense cyclic heating effects on thermo-fracture and thermal shock of solid tungsten and open-cell tungsten foam

We investigate here the effects of transient (cyclic) arc-jet plasma and laser heating on fracture behavior of W-foam and solid tungsten. The two key parameters that control the foam thermomechanical response are its density and mean cell size expressed in Pores Per Inch (PPI). Tungsten foam samples were fabricated with Chemical Vapor Deposition (CVD) with variety of PPI and relative density. These were tested under two types of qualitatively different conditions: (1) high-enthalpy arc-jet, and (2) high-power cyclic laser heating. None of the foam samples showed macroscopic through-thickness cracks. However, distributed micro-cracks were observed on ligaments and their triple junctions. Under the same loading conditions, W-foam and solid tungsten showed similar crack network pattern and characteristic length-scale. However, Crack Opening Displacement (COD) was twice as large in solid W as compared to W-foam. Foam samples that have been previously exposed to a low-pressure helium plasma showed significant changes in their surface forming nano-texture fuzz which was removed by subsequent testing in the arc-jet. Extensive fracture and re-crystallization were observed in the thin solid W disk that was fully-constrained from expansion. Thicker and fully-constrained solid W disks did not display recrystallization, grain growth, and extensive cracking. However, thicker disks that were free to expand showed some recrystallization and extensive through-thickness cracks due to less effective cooling and thus higher temperatures. Laser beam testing showed no visible damage formation at 0.19 GW/m2 and 0.38 GW/m2 for both low-density (23%) and high-density (43%) foams at low pulses (100-1000). Micro-cracks were observed after 10,000 pulses at 0.19 GW/m2 in both foams, and in low-density foam after 100,000 at 0.38 GW/m2. The nature of thermomechanical damage in W-foam exposed to extreme power (GW/m2) short-duration laser pulses was found to be qualitatively similar to that of high power (MW/m2) long-duration arc-jet.

Halides formation dynamics in nanosecond and femtosecond laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is an analytical technique based on the measurement of the emitted radiation coming from a laser-induced plasma (LIP) created after irradiation of a sample by a short-duration laser pulse. Research on molecular presence in LIPs has increased because the use of molecular emission has proven an encouraging way to improve LIBS abilities. LIPs are dynamic plasmas with fast time and spatial evolutions, in which atoms and molecules can follow different paths in their evolution and distribution. Molecular creation mechanisms within LIPs are still a challenging issue under investigation and the prevalence of some specific mechanisms are dependent on experimental conditions (sample nature, laser parameters, surrounding atmosphere…). In this work, different time and spatially solved experiments were carried out in ns- and fs-LIBS to investigate the dynamics of alkaline-earth (Ca) halide (F) diatomic molecule formation. Experiments were carried out on powdered CaF2 samples for both ns- and fs-LIBS. The effects of a gas flow (air, He, Ar) over the plume are investigated for ns-LIBS. Nebulization-modified ns-LIBS experiments in which the alkaline-earth element is externally added to the plasma plume as an aerosol were carried out on (C2F4) n samples. The spatial separation between atomic and molecular emission distribution was found to take place with and without external modifications over the ns-LIP. Behavior in fs-LIPs was determined to differ significantly from analogous experiments with nanosecond lasers, but temporal optimization remains the optimum method for molecular detection as spatial separation was not found to provide any remarkable advantage.

ATmospheric LIDar (ATLID): Pre-Launch Testing and Calibration of the European Space Agency Instrument That Will Measure Aerosols and Thin Clouds in the Atmosphere

ATLID (ATmospheric LIDar) is the atmospheric backscatter Light Detection and Ranging (LIDAR) instrument on board of the Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) mission, the sixth Earth Explorer Mission of the European Space Agency (ESA) Living Planet Programme. ATLID’s purpose is to provide vertical profiles of optically thin cloud and aerosol layers, as well as the altitude of cloud boundaries, with a resolution of 100 m for altitudes of 0 to 20 km, and a resolution of 500 m for 20 km to 40 km. In order to achieve this objective ATLID emits short duration laser pulses in the ultraviolet, at a repetition rate of 51 Hz, while pointing in a near nadir direction along track of the satellite trajectory. The atmospheric backscatter signal is then collected by its 620 mm aperture telescope, filtered through the optics of the instrument focal plane assembly, in order to separate and measure the atmospheric Mie and Rayleigh scattering signals. With the completion of the full instrument assembly in 2019, ATLID has been subjected to an ambient performance test campaign, followed by a successful environmental qualification test campaign, including performance calibration and characterization in thermal vacuum conditions. In this paper the design and operational principle of ATLID is recalled and the major performance test results are presented, addressing the main key receiver and emitter characteristics. Finally, the estimated instrument, in-orbit, flight predictions are presented; these indicate compliance of the ALTID instrument performance against its specification and that it will meet its mission science objectives for the EarthCARE mission, to be launched in 2023.

Target normal sheath acceleration with a large laser focal diameter

The dependence of the laser-driven ion acceleration from thin titanium foils in the Target Normal Sheath Acceleration (TNSA) regime on target and laser parameters is explored using two dimensional particle-in-cell simulations. The oblique incidence (θL=45°) and large focal spot size (w0=40μm) are chosen to take an advantage of quasi one-dimensional geometry of sheath fields and effective electron heating. This interaction setup also reveals low and achromatic angular divergence of a proton beam. It is shown that the hot electron temperature deviates from the ponderomotive scaling for short laser pulses and small pre-plasmas. This deviation is mainly due to the laser sweeping, as the short duration laser pulse each moment in time effectively heats only a fraction of a focal spot on the foil. This instantaneous partial heating results in an electron temperature deviation from the ponderomotive scaling and, thus, lower maximum proton energies than it could have been expected from the TNSA theory.

Development of a laser-induced breakdown spectroscopy system using a ceramic micro-laser for fiber-optic remote analysis

ABSTRACT In this study, a compact fiber-optic laser-induced breakdown spectroscopy (LIBS) system was developed using a microchip laser (MCL) with a monolithic Nd:YAG/Cr:YAG composite ceramic, for remote analysis of hazardous environments, such as nuclear reactor cores. Short duration laser pulses exhibiting a near-Gaussian beam profile were obtained. The output properties of the laser, such as pulse energy, repetition rate, temporal shape, and beam profile, were measured in view of their applicability to LIBS analysis and were found suitable for the purpose of this research. Spectra of zirconium metal were obtained, and signal intensity was further enhanced by applying multi-burst mode irradiation to the target. The results of this study reveal that the fiber-optic microchip-laser induced breakdown spectroscopy system is advantageous for efficient remote analysis of hazardous environments and is suitable for analyzing the inside of nuclear reactor cores.

Amplification of dissipative solitons with a polarisation-maintaining tapered fibre amplifier

The generation of strongly chirped dissipative solitons offers great opportunities for generating high-power laser pulses of short duration. We demonstrate the possibility of amplifying such pulses in a polarisation-maintaining tapered fibre having 32 μm mode diameter at the output end. Highly chirped pulses with a repetition rate of 14.3 MHz and a duration of 10 ps at 1055 nm centre wavelength are amplified in an all-fibre circuit to 2.11 μJ energy and compressed by diffraction gratings to a duration of ∼297 fs. Reducing the pulse repetition rate to 1 MHz allows a 16.5 μJ output pulse energy to be achieved without manifestations of SRS and self-phase modulation in the spectrum.

LIDAR TS for ITER core plasma. Part II: simultaneous two wavelength LIDAR TS

We have shown recently, and in more detail at this conference (Salzmann et al) that the LIDAR approach to ITER core TS measurements requires only two mirrors in the inaccessible port plug area of the machine. This leads to simplified and robust alignment, lower risk of mirror damage by plasma contamination and much simpler calibration, compared with the awkward and vulnerable optical geometry of the conventional imaging TS approach, currently under development by ITER. In the present work we have extended the simulation code used previously to include the case of launching two laser pulses, of different wavelengths, simultaneously in LIDAR geometry. The aim of this approach is to broaden the choice of lasers available for the diagnostic. In the simulation code it is assumed that two short duration (300 ps) laser pulses of different wavelengths, from an Nd:YAG laser are launched through the plasma simultaneously. The temperature and density profiles are deduced in the usual way but from the resulting combined scattered signals in the different spectral channels of the single spectrometer. The spectral response and quantum efficiencies of the detectors used in the simulation are taken from catalogue data for commercially available Hamamatsu MCP-PMTs. The response times, gateability and tolerance to stray light levels of this type of photomultiplier have already been demonstrated in the JET LIDAR system and give sufficient spatial resolution to meet the ITER specification. Here we present the new simulation results from the code. They demonstrate that when the detectors are combined with this two laser, LIDAR approach, the full range of the specified ITER core plasma Te and ne can be measured with sufficient accuracy. So, with commercially available detectors and a simple modification of a Nd:YAG laser similar to that currently being used in the design of the conventional ITER core TS design mentioned above, the ITER requirements can be met.

Local Enhancement of Lipid Membrane Permeability Induced by Irradiated Gold Nanoparticles.

Photothermal therapies are based on the optical excitation of plasmonic nanoparticles in the biological environment. The effects of the irradiation on the biological medium depend critically on the heat transfer process at the nanoparticle interface, on the temperature reached by the tissues, as well as on the spatial extent of temperature gradients. Unfortunately, both the temperature and its biological effects are difficult to be probed experimentally at the molecular scale. Here, we approach this problem using nonequilibrium molecular dynamics simulations. We focus on photoporation, a photothermal application based on the irradiation of gold nanoparticles by single, short-duration laser pulses. The nanoparticles, stably bound to cell membranes, convert the radiation into heat, inducing transient changes of membrane permeability. We make a quantitative prediction of the temperature gradient around the nanoparticle upon irradiation by typical experimental laser fluences. Water permeability is locally enhanced around the nanoparticle, in an annular region that extends only a few nanometers from the nanoparticle interface. We correlate the local enhancement of permeability at the nanoparticle-lipid interface to the temperature inhomogeneities of the membrane and to the consequent availability of free volume pockets within the membrane core.

Excitation of Lamb waves over a large frequency-thickness product range for corrosion detection

For corrosion detection, it is often desirable that a Lamb wave mode is highly sensitive to surface thinning and enjoys some degree of mode purity at a particular frequency. In view of this, this paper aims to generate a variety of Lamb wave modes over broad frequency bands to ensure an abundant supply of candidates for corrosion detection, and further, establish a strategy to find appropriate operation points efficiently and effectively. Firstly, a short-duration laser pulse is applied to generate Lamb waves over a large frequency-thickness product range. The selection of symmetric modes or anti-symmetric modes is obtained by addition or subtraction of signals captured by two identical transducers which are symmetrically coupled on both sides of the plate. Subsequently, the S0 mode at a non-dispersive frequency bandwidth is employed to improve the accuracy of the transmitter–receiver distance. Based on those, three selection criteria including mode separability, amplitude ratio and corrosion sensitivity, are presented to efficiently determine the suitable operation points (i.e., mode types and frequencies). The experimental results show that the simulated corrosion could be correctly detected and accurately localized at the chosen modes and frequencies.

Laser Shock Processing: Process Physics, Parameters, and Applications

Generally the fatigue life of the components can be improved by introducing compressive residual stresses up to certain depth from the surface. Laser shock processing (LSP) is one of the surface treatment processes to improve fatigue life of the components. When a short duration laser pulse with high power density (more than 1 GW/cm2) irradiates target surface coated with sacrificial layer in a confined medium generally, shock waves are produced. These shock waves deform the target surface plastically and strengthen the material surface up to certain depth to induce residual stresses. LSP improves many properties of the target material such as fatigue life, wear and corrosion resistance, hardness, and etc. This paper reviews the physics involved at all stages of the process generation of plasma at confined medium-material interface and laser induced shock waves, and generation of residual stresses. Also, the effect of various input parameters such as laser parameters, sacrificial layer material, sacrificial layer material thickness, and etc. on residual stress and hardness is discussed. Finally, the applications and challenges of the LSP are also discussed.

Investigations into localized re‐treatment of the retina with a 3‐nanosecond laser

Subvisual retinal lasers necessarily cause clinically invisible lesions, hence, they could intentionally or inadvertently be targeted at precisely the same or an overlapping location during repeat laser treatment. Herein, we investigated the structural integrity and cellular responses of localized re-treatment using a nanosecond laser (2RT) currently in trials for early age-related macular degeneration. Rats were randomly assigned to one of five groups: sham, subvisual 2RT, subvisual 2RT re-treatment, visual effect 2RT, visual effect 2RT re-treatment. Re-treatment groups were lasered on days 0 and 21; single laser groups were only lasered on day 21. All rats were euthanized at day 28 and eyes were then dissected and processed for immunohistochemistry. For re-treatment, the laser was targeted at precisely the same locations on both delivery occasions. Analytical endpoints included monitoring of retinal vascular integrity overlying lesions, investigation into any potential choroidal neovascularization, assessment of the RPE, quantification of collateral injury to photoreceptors or other neuronal classes, and delineation of glial reactivity. Repeat laser administration to rats caused ostensibly identical retinal-RPE-choroid responses to those obtained in age-matched rats that received only a single application. Specifically, 7 days after treatment, RPE cells were re-populating lesion sites. No obvious consistent differences were evident between the single and repeat laser groups. Moreover, repeat laser caused no (measurable) additive injury to photoreceptors or other retinal neuronal classes from single laser treatment. In re-lasered animals, there was no increase in microglial activity overlying and adjacent to lesion sites relative to single lasered rats. Finally, there was no evidence of choroidal neovascularization after repeat laser treatment. The overall results provide a measure of confidence that re-treatment of patients with 2RT should not provide any additional risk of developing visual scotomas, choroidal neovascularizations, or inflammatory events. Indeed, the collated results indicate that the metabolic and structural disruption to the RPE-retina caused by short pulse duration laser treatment is resolved within a short time frame such that re-treatment elicits a phenotype indistinguishable from single treatment. Lasers Surg. Med. 48:602-615, 2016. © 2016 Wiley Periodicals, Inc.

Control of the photoelectron dynamics for the effective conversion of short-pulse, frequency-modulated optical radiation into X-ray radiation

We report a theoretical investigation of high-order harmonic generation (HHG) in the ionisation of atoms by nonlinear frequency-modulated laser pulses of short duration. It is shown that the reduction in the instantaneous value of the laser pulse frequency can lead to a significant increase in the width of the plateau in the HHG spectrum. We have found optimal parameters of frequency modulation at which photons with energies of are efficiently generated at a relatively low laser-pulse intensity. High HHG efficiency at optimal parameters is explained by the peculiarities of atomic ionisation dynamics and acceleration of photoelectrons by a frequency-modulated laser field.

Experimental analysis on the effects of DC arc discharges at various flow regimes

This paper addresses the control of the boundary layer on a compression ramp by means of DC electrical arc discharges. The development and realization of the control system are first described and then assessed in the wind tunnel. The objective of the research was to control the supersonic flow using the minimum amount of energy. The array of electrodes was located at the base of a ramp, where a low momentum flow develops. The electrical discharge was generated by a custom designed electronic facility based on high-voltage ignition coils. The slanted tungsten electrodes were insulated by mounting them in a ceramic support. The discharge evolution was studied through high-speed flow visualizations, while electrical measurements at the high-voltage section of the circuitry allowed to estimate the energy release. The development of a high-speed short exposure Schlieren imaging technique, based on a very short duration laser pulse illumination and a double shot CCD camera, allowed to observe the macroscopic effects associated with the arc establishment between the electrodes (glow, sound wave and heat release). Due to the long residence time, the thermal perturbation spread along the streamwise direction. Cross correlation of Schlieren images with short time separation revealed that in supersonic conditions, the discharges led to an overall acceleration of the flow field underneath the oblique shock wave.

Photodetachment of Si− by mid-infrared few-cycle femtosecond laser pulses

The Kelydsh model originally developed for infinitely long periodic laser pulses is extended to a spin polarized model to consider direct photodetachment of negative ions with half-filled np3 valence shells by few-cycle laser pulses without taking into account rescattering effects. The theory is applied to study above threshold detachment of Si− negative ions by intense short infrared pulses but is expected to be inherent of any half-filled p subshell negative ion. Photoelectron momentum maps of the direct electrons of Si− are presented. These are found to exhibit a distinctive threshold structure of concentric elliptical rings where the separation of these rings is dependent on the quantized frequency components of the few-cycle laser pulse. Photoelectron angular distributions (PADs) predicted by short duration laser pulses are calculated and threshold effects associated with channel closings are observed as the laser-field frequency and intensity pass through critically induced ponderomotive potential channel closures. Enhanced structures of the PADs in the vicinity near threshold show that the detachment process does not follow the Wigner law. This is a because the photon energy is not conserved in a few-cycle pulse in contrast to a long periodic pulse. In the energy spectra interference effects are observable as multiphoton peaked structures whose positions are dependent on the ponderomotive threshold shifts determined by the nature of the time-dependent vector potential. Additionally we show that carrier envelope phase effects manipulate all emission spectra considered.

Influence of consecutive picosecond pulses at 532 nm wavelength on laser ablation of human teeth

The interaction of 40 ps pulse duration laser emitting at 532 nm wavelength with human dental tissue (enamel, dentin, and dentin–enamel junction) has been investigated. The crater profile and the surface morphology have been studied by using a confocal auto-fluorescence microscope (working in reflection mode) and a scanning electron microscope. Crater profile and crater morphology were studied after applying consecutive laser pulses and it was found that the ablation depth increases with the number of consecutive pulses, leaving the crater diameter unchanged. We found that the thermal damage is reduced by using short duration laser pulses, which implies an increased retention of restorative material. We observe carbonization of the irradiated samples, which does not imply changes in the chemical composition. Finally, the use of 40 ps pulse duration laser may become a state of art in conservative dentistry.

Phase diagram for sodium-chloride crystals at high pressures produced by laser pulses of short duration

Phase diagram for sodium-chloride crystals at high pressures produced by laser pulses of short duration