Double-pulse Delay Research Articles

Process simulation of laser shock-based disassembly of adhesively bonded composite/metallic structural parts: a numerical upscaling

ABSTRACT In previous works, both experimental and numerical investigations have shown the potential of the laser shock technique for disassembling adhesively bonded composite/metal coupons. This study extends this concept to upscale the method, focusing on the full disassembly of a foreign object damage (FOD) panel designed to replicate an aircraft engine fan blade. Utilizing LS-Dyna explicit FE software, the numerical simulation incorporates various material models: a progressive damage model for the 3D CFRP substrate, Johnson–Cook plasticity model combined with Grüneisen equation of state for the titanium layer, and a cohesive zone model with a bilinear traction separation law for the adhesive layer. Investigating the FOD panel’s inclined geometry, a preliminary analysis examines the impact of inclination angles on shock wave propagation and back face velocity, revealing minimal alterations. Initial simulations are conducted to determine double pulse delay times and evaluate spot location effects. Automation of the multi-step process simulation is achieved through a Python script. After approximately 30 shots, complete disassembly of the FOD strip is achieved, with observed damage primarily limited to matrix cracking in the composite substrate. Numerical findings support the efficacy of the double-shot laser shock method for disassembling adhesively bonded metallic/CFRP components, contingent upon appropriate experimental arrangements.

Reducing self-absorption effect by double-pulse combination in laser-induced breakdown spectroscopy

The method of double-pulse laser-induced breakdown spectroscopy is usually employed to enhance the spectral signal intensity. However, in this study, double-pulse laser-induced breakdown spectroscopy is adopted to investigate the effect of the self-absorption reduction of the spectrum. This research explored that the influence of the change of the gas environment generated by the first laser beam on the self-absorption effect of the plasma spectrum by the second laser beam. Especially despite the different combinations of laser energy, for the three elements of Cu, Mn and Ni, the weakest spectral self-absorption effect can be obtained when the double-pulse delays are around 80 μs, 100 μs, and 110 μs, respectively. In addition, this paper also found that when the energy of the first laser beam is unchanged, the spectral self-absorption effect has a strong correlation with the double-pulse delay, and has a weak correlation with the change of the second laser energy.

Throughput Improvement in Femtosecond Laser Ablation of Nickel by Double Pulses.

In this study, femtosecond laser double pulses were tested to improve their nickel ablation efficiency. The experimental results indicated that compared with single pulses, double pulses with different delay times generated craters with larger diameters and depths. The results obtained for three sets of double pulses with different energy ratios indicated that double pulses with an energy ratio of 1:9 had the highest ablation efficiency, followed by those with energy ratios of 2:8 and 5:5. The double pulses with the aforementioned three energy ratios achieved the maximum ablation efficiency when the delay time was 3–4 ps. Compared with single pulses, double pulses with an energy ratio of 1:9 generated craters with an up to 34% greater depth and up to 14% larger diameter. In addition, an interference effect was observed with a double pulse delay time of 0 ps, which has seldom been reported in the literature. The double pulses were simulated using the two-temperature model. The simulation results indicated that double pulses with an energy ratio of 1:9 with a delay time of 4 ps can perform the strongest ablation. These simulation results are in line with the experimental results.

Nanopatterning of silicon via the near-field enhancement effect upon double-pulse femtosecond laser exposure.

Utilizing the near-field enhancement effect of a polystyrene microsphere, direct ablation of nanohole arrays by a temporal-shaping femtosecond (fs) laser pulse is presented. The nanohole arrays, which are circular, regular, and free of cracks, were processed without extra post-processing, and their average diameter decreased gradually, as the double-pulse delay increased until 2500 fs. The simulated results by a plasma model and finite difference time domain solution demonstrate that the size decrease of the structure is attributed to the increase of the ablation threshold of silicon. Through fs laser near-field fabrication, the FWHM of nanoholes can be reduced to approximately 50 nm (λ/16) and even to 23 nm when using the second harmonic laser at a wavelength of 400 nm.

Martensitic transformation in temporally shaped femtosecond laser shock peening 304 steel

Laser shock peening (LSP) plays an important role in simultaneously strengthening and toughening materials for many engineering applications, such as key structures of aircraft and ship. This work focused on the application of temporally shaped femtosecond (fs) laser in shock peening 304 stainless steel. In our experiment, a single-fs laser pulse was divided by Michelson interferometer to generate two subpulses with a time delay ranging from 5 to 50 picoseconds (ps), and then the double-pulse fs laser was focused by an objective and hit the stainless steel surface. We revealed the mechanism of fs-LSP theoretically by a one-dimensional microscale heat transfer and thermoelastic model and experimentally by the time-resolved shadowgraph, X-ray diffraction, and election backscattering diffraction. When the first subpulse hit the material, plasma gradually formed and then the plasma electron density and temperature decreased. More energy deposited into the material because of the higher laser transmission caused by the decreased plasma electron density. The lower energy transfer rate caused by the decreased plasma electron temperature, along with the lower stress propagation rate caused by the plasma expansion, hindered the energy and stress transfer into the material. Herein, the optimal mechanical response achieved at the double-pulse delay of 20 ps was beneficial to martensitic transformation, and it ultimately induced more than a 20% improvement of hardness. Our method provided insight for structural transformation and surface strengthen by controlling the electron dynamics, which holds significance not only for strengthening 304 steel, but also for other extensive material systems.

Characteristics of Bubble Oscillations During Laser-Activated Irrigation of Root Canals and Method of Improvement.

Background and ObjectivesLaser‐activated irrigation of dental root canals is being increasingly used as its efficacy has been shown to be superior compared with conventional techniques. The method is based on laser‐initiated localized fluid evaporation and subsequent rapid bubble expansions and collapses, inducing microfluid flow throughout the entire volume of the cavity. The irrigation efficacy can be further improved if optimally delayed “SWEEPS” double laser pulses are delivered into the canal. This study aims to show that the irrigation efficacy, as measured by the induced pressure within the canal, is related to the double pulse delay, with the maximal pressure generated at an optimal delay. The second aim is to find a method of determining the optimal delay for different cavity dimensions and/or laser parameters.Study Design/Materials and MethodsExperiments were made in transparent models of root canals where Er:YAG laser (λ = 2.94 μm, pulse duration t p = 25 or 50 microseconds, and pulse energies up to E L = 40 mJ) was used with a combination of cylindrical and conical fiber‐tip geometries (diameters 400 and 600 µm). High‐speed photography (60,000 fps) and average pressure measurements inside the canal were used for process characterization.ResultsThe results show that a pressure amplification of more than 1.5 times occurs if the laser pulse delay approximately coincides with the bubble oscillation time. Correlations between normalized oscillation time and canal diameter for a wide range of laser pulse energies (R 2 = 0.96) and between the average pressure within the canal and the bubble oscillation periods (R 2 = 0.90) were found. A relationship between the bubble oscillation time and the diameter of the treated cavity was found depending on the bubble oscillation time in an infinite fluid reservoir.ConclusionsThe bubble oscillation time within a constrained volume can be determined based on the known oscillation time in infinite space, which offers a fast and simple solution for optimization of the laser parameters. These findings enable determination of optimal conditions for shock wave generation, and improvement of root canal irrigation at the same dose of laser energy input, leading to improved treatment efficacy and safety. Lasers Surg. Med. © 2020 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals, Inc.

Mode selective excitation of terahertz vibrations in single crystalline rubrene.

Organic molecular crystals have a variety of low frequency vibrational modes composed of intra- and inter-molecular oscillations. They are mixed intricately in the terahertz (THz) region. We are interested in the controllability of the vibrational energy distribution among such THz vibrational modes based on the femtosecond double-pulse excitation scheme. Single crystalline rubrene is prepared by physical vapor transport. The optical response of vibrational modes in the electric ground state of rubrene is detected by the ultrafast pump-probe reflectivity measurement at 90 K. Three oscillation modes at 3.20, 3.67, and 4.18 THz are detected, and we demonstrate selective enhancement and depletion of each mode by properly tuning the double-pulse delay. The amplitude of the selected vibrational mode is modulated between 0.149 and 1.87, where 1.0 corresponds to the amplitude excited with a single pump pulse. The double-pulse delay dependence of the observed vibrational amplitude is simulated based on the classical driven harmonic oscillator model, and the results reasonably reproduce our experimental signals. Such selective manipulation of the vibrational amplitude can be a potential tool to investigate the vibronic and electron-phonon couplings which plays an important role for the charge transport characteristics and various optoelectronic properties in organic molecular crystals.

Drilling of aluminum and copper films with femtosecond double-pulse laser

Aluminum and copper films are drilled with femtosecond double-pulse laser. The double-pulse delay is scanned from −75ps to 90ps. The drilling process is monitored by recording the light transmitted through the sample, and the morphology of the drilled holes is analyzed by optical microscopy. It is found that, the breakthrough time, the hole evolution during drilling, the redeposited material, the diameters of the redeposited area and the hole, change as functions of double-pulse delay, and are different for the two metals. Along the double-pulse delay axis, three different time constants are observed, a slow one of a few tens of picoseconds, a fast one of a few picoseconds, and an oscillation pattern. Results are discussed based on the mechanisms of plasma shielding, electron–phonon coupling, strong coupling of laser with liquid phase, oxidation of aluminum, laser induced temperature and pressure oscillations, and the atomization of plume particles.

Femtosecond laser high-efficiency drilling of high-aspect-ratio microholes based on free-electron-density adjustments.

We studied the micromachining of high-aspect-ratio holes in poly(methylmethacrylate) using a visible double-pulse femtosecond laser based on free-electron-density adjustments. Hole depth and aspect ratio increased simultaneously upon decreasing the wavelength in the visible-light zone. When the pulse energy reached a high level, the free-electron density was adjusted by using a double-pulse laser, which induced fewer free electrons, a lower reflectivity plasma plume, and more pulse energy deposition in the solid bottom. Thus, the aspect ratio of the hole was improved considerably. At a moderate pulse energy level, a 1.3-1.4 times enhancement of both the ablation depth and the aspect ratio was observed when the double-pulse delay was set between 100 and 300fs, probably due to an enhanced photon-electron coupling effect through adjusting the free-electron density. At a lower pulse energy level, this effect also induced the generation of a submicrometer string. In addition, the ablation rate was improved significantly by using visible double pulses.

The influence of double pulse delay and ambient pressure on femtosecond laser ablation of silicon

The effects of double pulse delay time and ambient pressure on femtosecond laser ablation of Si are studied by means of photoluminescence and morphology analysis. Detailed scan of double pulse delay is performed at vacuum (2 Pa) and atmospheric pressure, and detailed scan of pressure is performed at three double pulse delay times of 0.2 ps, 53.13 ps and 106.47 ps. It is found that, at various fluences, photoluminescence intensity and morphology change as functions of both double pulse delay and ambient pressure. Especially, the shape of the splashing droplets also changes at different experimental conditions, indicating higher or lower sample temperature. The observations are explained by the efficiently energy coupling between the second pulse and the liquid layer produced by the first one, and the pressure dependent energy coupling between plasma and liquid phase as well as ambient gas. • The splashing droplet shape changes at different experimental conditions. • Photoluminescence, crater depth and width change as a function of ambient pressure. • Different trends of photoluminescence as a function of delay at various fluences.

Anisotropy modulations of femtosecond laser pulse induced periodic surface structures on silicon by adjusting double pulse delay.

We demonstrate that the polarization-dependent anisotropy of the laser-induced periodic surface structure (LIPSS) on silicon can be adjusted by designing a femtosecond laser pulse train (800 nm, 50 fs, 1 kHz). By varying the pulse delay from 100 to 1600 fs within a double pulse train to reduce the deposited pulse energy, which weakens the directional surface plasmon polarition (SPP)-laser energy coupling based on the initial formed ripple structure, the polarization-dependent geometrical morphology of the LIPSS evolves from a nearly isotropic circular shape to a somewhat elongated elliptical shape. Meanwhile, the controllable anisotropy of the two-dimensional scanned-line widths with different directions is achieved based on a certain pulse delay combined with the scanning speed. This can effectively realize better control over large-area uniform LIPSS formation. As an example, we further show that the large-area LIPSS can be formed with different scanning times under different pulse delays.

Ablation area quasiperiodic oscillations in semiconductors with femtosecond laser double-pulse delay.

A surprising repeatable phenomenon regarding semiconductor ablation area changes has been discovered. Irradiated by femtosecond double pulses, the ablation area quasiperiodically oscillates as the pulse delay increases from 0 to 1ps at a material-dependent fluence range. In contrast, the ablation area monotonically decreases as the pulse delay increases beyond 1ps or if the total fluence increases close to or beyond the single-shot threshold. Similar unexpected patterns of area quasiperiodic oscillations with the double-pulse delay are observed in various semiconductors, including Ge, Si, GaAs, and ZnO. The comparison study shows the same phenomenon in Au-plated ZnO. Yet, its oscillation periods are shorter and more stable than those in bulk ZnO, which implies that the localized carrier density is the key factor in oscillation periods.

Rippled area formed by surface plasmon polaritons upon femtosecond laser double-pulse irradiation of silicon

The formation of near-wavelength laser-induced periodic surface structures (LIPSS) on silicon upon irradiation with sequences of Ti:sapphire femtosecond laser pulse pairs (pulse duration 150 fs, central wavelength 800 nm) is studied theoretically. For this purpose, the nonlinear generation of conduction band electrons in silicon and their relaxation is numerically calculated using a two-temperature model approach including intrapulse changes of optical properties, transport, diffusion and recombination effects. Following the idea that surface plasmon polaritons (SPP) can be excited when the material turns from semiconducting to metallic state, the "SPP active area" is calculated as function of fluence and double-pulse delay up to several picoseconds and compared to the experimentally observed rippled surface areas. Evidence is presented that multi-photon absorption explains the large increase of the rippled area for temporally overlapping pulses. For longer double-pulse delays, relevant relaxation processes are identified. The results demonstrate that femtosecond LIPSS on silicon are caused by the excitation of SPP and can be controlled by temporal pulse shaping.

Formation of laser-induced periodic surface structures on fused silica upon multiple parallel polarized double-femtosecond-laser-pulse irradiation sequences

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica with multiple irradiation sequences of parallel polarized Ti:sapphire femtosecond laser pulse pairs (160fs pulse duration, 800nm central wavelength) was studied experimentally. For that purpose, a Michelson interferometer was used to generate near-equal-energy double-pulse sequences allowing the temporal pulse delay between the parallel-polarized individual fs-laser pulses to be varied between 0 and 40ps with ∼0.2ps temporal resolution. The surface morphologies of the irradiated surface areas were characterized by means of scanning electron and scanning force microscopy. In the sub-ps delay range a strong decrease of the LIPSS periods and the ablation crater depths with the double-pulse delay was observed indicating the importance of the laser-induced free-electron plasma in the conduction band of the solids for the formation of LIPSS.

Periodic nanoscale structures on polyimide surfaces generated by temporally tailored femtosecond laser pulses

A strong influence of different pulse durations and double pulse delay times on the formation of periodic surface structures on polyimide were observed employing ultrashort laser pulses tailored on a sub-picosecond and picosecond time scale. Multi-photon, defect-related excitation mechanisms and thermal expansion of the polymer lattice correlated to a loss of long range order and polarisation memory were considered.

Ultrashort double pulse laser ablation of metals

The peculiarities of laser microablation and plasma re-heating with ultrashort (50 fs–10 ps) double pulses were studied. Three regimes of laser/metal target interaction were observed as a function of double-pulse delay: without any plasma shielding (Δt12≤1 ps), intermediate (1≤Δt12≤10 ps) and with total plasma shielding (from 10 to 250 ps). The optimal plasma re-heating regime with the highest plasma intensity and reproducibility was determined to correspond to the double pulses delay of 100–200 ps. The pulses in the 50 fs–2 ps range were observed to give approximately the same ablation efficiency with a metal target.