Nanosecond Laser Research Articles

Inspired square micro-texture structures designed by nanosecond laser sculpture for enhancing adhesion strength of plasma spraying Si-HfO2 bond coat on SiC substrates

In order to enhance the adhesion strength between the Si-HfO2 bond coat and substrate, square micro-textures inspired by straw-checkered sand fixation are fabricated using nanosecond laser sculpture. The adhesion strength of the laser surface texture (LST) sample is up to 27.31 ± 3.34 MPa, which is more than 200 % higher than the conventional grit-blasted one (8.12 ± 0.70 MPa). The promoted interface bonding is attributed to the following three aspects: First, the mechanical anchoring established by micro-texture positively improves the adhesion strength. Second, the LST diminishes the contact angle to 16.5 ± 1.7°, improving the wetting of the SiC substrate. Third, the high-pressure plasma is implemented to produce enormous compressive stress (205.6 ± 14.6 MPa) on the SiC substrate surface, greatly aiding substrate/coating adhesion. The above three mechanisms work synergistically to enable the LST treatment to exhibit better adhesion than the conventional grit-blasted treatment on SiC substrate.

Conformer-Selective Photoelectron Circular Dichroism.

Conformational flexibility and chirality both play a key role in molecular recognition. It is therefore very useful to develop spectroscopic methods that simultaneously probe both properties. It has been theoretically predicted that photoelectron circular dichroism (PECD) should be very sensitive to conformational isomerism. However, experimental proof has been less forthcoming and only exists for a very few favorable cases. Here, we present a new PECD scheme based on resonance-enhanced two-photon ionization (RE2PI) using UV/Vis nanosecond laser excitations. The spectral resolution obtained thereby guarantees conformer-selectivity by inducing resonant conformer-specific ππ* S1←S0 transitions. We apply this experimental scheme to the study of chiral 1-indanol, which exists in two conformers linked by a ring inversion and defined by the position of the hydroxyl group, namely axial and equatorial. We show that the PECD of the equatorial and axial forms considerably differ in sign, magnitude and shape. We also discuss the influence of the total ionization energy, vibronic excitation of intermediate and final states, and relative polarization of the excitation and ionization lasers. Conformer-specificity adds a new dimension to the applications of PECD in analytical chemistry addressing now the general case of floppy systems.

Conformer‐Selective Photoelectron Circular Dichroism

AbstractConformational flexibility and chirality both play a key role in molecular recognition. It is therefore very useful to develop spectroscopic methods that simultaneously probe both properties. It has been theoretically predicted that photoelectron circular dichroism (PECD) should be very sensitive to conformational isomerism. However, experimental proof has been less forthcoming and only exists for a very few favorable cases. Here, we present a new PECD scheme based on resonance‐enhanced two‐photon ionization (RE2PI) using UV/Vis nanosecond laser excitations. The spectral resolution obtained thereby guarantees conformer‐selectivity by inducing resonant conformer‐specific ππ* S1←S0 transitions. We apply this experimental scheme to the study of chiral 1‐indanol, which exists in two conformers linked by a ring inversion and defined by the position of the hydroxyl group, namely axial and equatorial. We show that the PECD of the equatorial and axial forms considerably differ in sign, magnitude and shape. We also discuss the influence of the total ionization energy, vibronic excitation of intermediate and final states, and relative polarization of the excitation and ionization lasers. Conformer‐specificity adds a new dimension to the applications of PECD in analytical chemistry addressing now the general case of floppy systems.

Superhydrophobic Surfaces with Excellent Ice Prevention and Drag Reduction Properties Inspired by Iridaceae Leaf.

The anti-icing and drag-reduction properties of diverse microstructured surfaces have undergone extensive study over the past decade. Nonetheless, tough environments enforce stringent demands on the composite characteristics of superhydrophobic surfaces (SHS). In this study, fresh composite structures were fabricated on a metal substrate by nanosecond laser machining technology, drawing inspiration from the hardy plant Iridaceae. The prepared sample surface mainly consists of a periodic microrhombus array and irregular nanosheets. To comprehensively investigate the effect of its special structure on surface properties, three surfaces with different sizes of rhombic structures were used for comparative analysis, and the results show that the SH-S2 sample is optimal. This can significantly delay the freezing time by an impressive 1404 s at -10 °C while revealing the sample surface anti-icing strategy. In addition, the rheological experiments determined over 300 μm of slip length for the SH-S2 sample, and the drag reduction rate of the surface reaches nearly 40%, which is well aligned with the results of the delayed icing experiments. Finally, the mechanical durability of the SH-S2 surface was investigated through scratch damage, sandpaper abrasion, reparability trials, and icing and melting cycle tests. This research presents a new approach and methodology for the application of SHS on polar ship surfaces.

Laser-assisted synthesis of nano-hydroxyapatite and functionalization with bone active molecules for bone regeneration

The main goal of bone tissue engineering research is to replace the allogenic and autologous bone graft substitutes that can promote bone repair. Owing to excellent biocompatibility and osteoconductivity, hydroxyapatite is in extensive research and high demand for both medical and non-medical applications. Although various methods have been developed for the synthesis of hydroxyapatite, in the present study we have shown the use of nanosecond laser energy in the wet precipitation method of nano-hydroxyapatite (nHAP) synthesis without using ammonium solution or any other chemicals for pH maintenance. Here, the present study aimed to fabricate the nanohydroxyapatite using a nanosecond laser. The X-ray diffraction and Fourier transform infrared spectroscopy have confirmed the hydroxyapatite formation under laser irradiation in less time without aging. A transmission electron microscopy confirmed the nano size of synthesized nHAP, which is comparable to conventional nHAP. The length and width of the laser-assisted nHAP were found to be in the range of 50–200 nm and 15–20 nm, respectively, at various laser parameters. The crystallite size obtained by Debye Scherrer formulae was found to be in the range of ∼ 16–36 nm. In addition, laser-assisted nHAP based composite cryogel (nanohydroxyapatite/gelatin/collagen I) was synthesized and impregnated with bioactive molecules (bone morphogenic protein and zoledronic acid) that demonstrated significant osteogenic potential both in vitro in cell experiment and in vivo rat muscle pouch model (abdomen and tibia muscles). Dual-energy X-ray analysis, micro-CT, and histological analysis confirmed ectopic bone regeneration. Micro-CT based histomorphometry showed a higher amount (more than 10-fold) of mineralization for animal groups implanted with composite cryogels loaded with bioactive molecules compared to only composite cryogels groups. Our findings thus demonstrate a controlled and rapid synthetic method for the synthesis of nHAP with various physical, chemical, and biological properties exhibited as comparable to conventionally synthesized nHAP.

Application of pulsed laser ablation thermal model in nanosecond pulsed laser removal of the epoxy resin paint film

The effect of laser surface paint removal depends on the mechanism of paint removal. The mechanism of nanosecond laser removal of epoxy resin paint film on aluminum alloy surfaces was studied theoretically and experimentally. Firstly, a laser thermal paint removal model was established based on the heat conduction equation, Hertz-Knudsen equation, and Hooke's law. The adsorption relationship between the paint film and the aluminum alloy substrate molecules and the adsorption force between the paint film and the substrate were analyzed and calculated. A laser thermal paint removal model was solved using COMSOL finite element software. By analyzing the changes in temperature and thermal stress with laser fluence, we obtained the theoretical threshold for laser paint removal. Subsequently, experimental research was conducted on laser paint removal. Through theoretical and experimental studies, it was found that the paint removal threshold of nanosecond pulse laser is 0.47 J/cm2. The primary mechanism of nanosecond laser paint removal is thermal stress stripping. When the laser fluence is low, the nanosecond pulse laser can only ablate the paint on the surface, and the thermal stress between the paint film and the substrate makes it difficult to remove the paint film. As the laser fluence increases, the nanosecond laser will ablate the surface paint film, and the thermal stress between the paint film and the substrate will cause the stripping of the underlying paint film and the substrate. The primary mechanism of laser paint removal is stripping.

Surface damage induced by micropores in transparent ceramics under nanosecond laser irradiation.

The increasing use of transparent ceramics in laser systems presents a challenge; their low damage threshold has become a significant impediment to the development of powerful laser systems. Consequently, it is imperative to undertake research into the damage sustained by these materials. Micropores, the most common structural defects in transparent ceramics, inevitably remain within the material during its preparation process. However, the relationship between the density and size of these micropores and their impact on nanosecond laser damage threshold and damage evolution remains unclear. In this study, we utilize the annealing process to effectively manage the density and size of micropores, establishing a correlation between micropores in relation to damage thresholds. This study confirms for the first time that micropores significantly contribute to laser damage, comparing and analyzing the damage morphology characteristics of both front and rear surfaces of transparent ceramics. It also presents, potential mechanisms that may contribute to these differences in damage. This paper offers guidance for controlling micropores during the preparation and processing of transparent ceramics with high laser damage thresholds. The findings are expected to further improve the anti-nanosecond laser damage capabilities of transparent ceramics.

Processing characteristics and ablation mechanism of CMC-SiCf/SiC by rotational thirteen-beam coupling nanosecond laser

Silicon carbide fiber reinforced silicon carbide ceramic matrix composite (CMC SiCf/SiC) has the advantages of high temperature resistance, corrosion resistance, high strength, low density, and oxidation resistance. It is a high-performance advanced material in aerospace, energy and other fields. However, due to its high hardness, brittleness and anisotropy of fiber structure, the research on application-oriented high-quality processing technology has become a hot issue. In this paper, a new process of rotational thirteen-beam coupling nanosecond laser processing of CMC-SiCf/SiC was studied. The rotational thirteen-beam coupling ablation experiments were carried out on CMC-SiCf/SiC workpiece. A laser beam is divided into thirteen beams and rotated around the beam center by using a beam splitter element. The morphology, structure and composition of ablation holes at different speeds were observed and analyzed by laser confocal microscope, scanning electron microscope, and X-ray energy dispersive spectrometer. It is found that the characteristics of rotational beam coupling have a significant effect on the laser ablation morphologies. The results show that the ablation region of CMC-SiCf/SiC is elliptical due to the fast heat transfer of fibers. The energy density distribution of the waist section of the Gaussian like beam after beam coupling is similar to the “W" shape, and the central energy is lower than the surrounding energy, and the ablation holes also show a similar “W" shape. With the increase of rotational speed, the hole depth first decreased and then increased, the hole diameter and roundness first increased and then decreased, and the hole section was “W" shaped. The ablation area can be divided into five parts: the white granular SiO2 solid coverage area, the recast layer, the lower recast layer, the molten layer, and the heat affected zone. The rotational characteristics will make the reaction intensity of the whole ablation area more uniform in the circumferential direction. The formation mechanism and element distribution of each part were explored. The research work in this paper will provide a new technical way for the high-performance application of CMC-SiCf/SiC.

Novel Combustion Instability Diagnosis Method With Upstream Pulsation of Repetitive Laser-Induced Plasmas

Abstract In this experimental study, we are presenting the ability of laser-induced plasmas with successive pulsation to identify combustion instabilities (CI) of a premixed lab-scale combustor. An acoustic disturbance equivalent to a shockwave perturbation is generated in the main air supply line of a swirled injector prior to the fuel addition by focusing nanosecond laser pulses of 1.6 W average power at 10 Hz. The shockwaves are attenuated to be strong pressure waves when reaching the combustor and impact the pressure field for short periods. After plasma breakdowns, the system returns back to its original state after 4 ms once the added acoustic energy has been fully dissipated. Given a set geometry, it is observed that the laser-induced breakdown amplifies the characteristic frequency peaks of the combustor system when actuated in cold flow. Furthermore, when applied to reacting flows, the pulsating acoustic perturbations impact the pressure fluctuation in the combustor, e.g., reducing the amplitude of the primary characteristic frequency peak at certain conditions. The identification of the main instability modes thanks to the plasma shockwave provides proof of the potential use of this novel diagnosis strategy in various and complex combustion systems.

Photochemistry of (n-Bu4N)2[Pt(NO3)6] in acetonitrile.

Photochemistry of the (n-Bu4N)2[Pt(NO3)6] complex in acetonitrile was studied by means of stationary photolysis and nanosecond laser flash photolysis. The primary photochemical process was found to be an intramolecular electron transfer followed by an escape of an •NO3 radical to the solution bulk. The spectra of two successive Pt(III) intermediates were detected in the microsecond time domain, and their spectral and kinetic characteristics were determined. These intermediates were identified as PtIII(NO3)52- and PtIII(NO3)4- complexes. Disproportionation of Pt(III) species resulted in formation of final Pt(II) products.

Versatile design for temporal shape control of high-power nanosecond pulsed fiber laser amplifier.

This research proposed a novel pulse-shaping design for directly shaping distorted pulses after the amplification. Based on the principle of the design we made a pulse shaper. With this pulse shaper, we successfully manipulate the pulse's leading edge and width to achieve an 'M'-shaped waveform in an amplification system. Comparative experiments were conducted within this system to compare the output with and without the integration of the pulse shaper. The results show a significant suppression of the nonlinear effect upon adding the pulse shaper. This flexible and effective pulse shaper can be easily integrated into a high-power all-fiber system, supplying the capability to realize the desired output waveform and enhance the spectral quality.

Alternate deposition and remelting microdroplets via single laser for printing low-defect and high-performance metal micropillars

Laser-induced forward transfer (LIFT) has emerged as a versatile technique for printing high-resolution metal microstructures. However, a common drawback of this method is the inadequate coalescence of the deposited metal microdroplets, which results in inferior electrical and mechanical properties. This paper proposes a novel approach for fabricating high-performance metal micropillars using a single-pulsed laser to alternately deposit and remelt metal microdroplets. Specifically, an ultraviolet nanosecond laser was used to induce the deposition of copper microdroplets, forming a patterned powder bed with high resolution. Subsequently, a laser pulse train was applied to fuse the patterned powder bed. The results showed that voids and microdroplet delamination were eliminated in the printed copper micropillars, whose yield strength and elastic modulus increased threefold, approaching 63% of those of the bulk metal. The remelting behavior of the deposited microdroplets was elucidated by modelling and analysing the thermal accumulation effects of a laser pulse train. A remelting map was proposed, including the non-melting, remelting, and vaporizing regimes. According to the depth of melt pool, the evolutions of morphology and microstructure in the depositing and remelting process were elucidated. Hence, this study advances the LIFT process for fabricating high-performance metal microstructures.

Efficient Magnetic Vortex Acceleration by femtosecond laser interaction with long living optically shaped gas targets in the near critical density plasma regime

We introduce a novel, gaseous target optical shaping laser set-up, capable to generate short scale length, near-critical target profiles via generated colliding blast waves. These profiles are capable to maintain their compressed density for several nanoseconds, being therefore ideal for laser-plasma particle acceleration experiments in the near critical density plasma regime. Our proposed method overcomes the laser-target synchronization limitations and delivers energetic protons, during the temporal evolution of the optically shaped profile, in a time window of approximately 2.5 ns. The optical shaping of the gas-jet profiles is optimised by MagnetoHydroDynamic simulations. 3D Particle-In-Cell models, adopting the spatiotemporal profile, simulate the 45 TW femtosecond laser plasma interaction to demonstrate the feasibility of the proposed proton acceleration set-up. The optical shaping of gas-jets is performed by multiple, nanosecond laser pulse generated blastwaves. This process results in steep gradient, short scale length plasma profiles, in the near critical density regime allowing operation at high repetition rates. Notably, the Magnetic Vortex Acceleration mechanism exhibits high efficiency in coupling the laser energy into the plasma in the optically shaped targets, resulting to collimated proton beams of energies up to 14 MeV.

Nanosecond laser damage of 532 nm thin film polarizers evaluated by different testing protocols

Different laser damage testing protocols, including 1-on-1, S-on-1, and Raster Scan, were conducted on 532 nm polarizers to evaluate the damage resistance and mechanisms. A comparison of the laser-induced damage thresholds (LIDTs) revealed different damage characteristics and major contributions to film failure for different polarizations. For P polarization, the LIDTs for the three test protocols were nearly the same because of the stabilization of two typical damage morphologies of flat-bottomed pits and mussel damage pits. For S polarization, the LIDTs for multiple pulses were lowest, where absorptive defect-induced damage was revealed. The damage characteristics of 532 nm polarizers and 1064 nm polarizers have been also compared. The mussel damage pits, which are similar to the typical damage morphologies of ultraviolet (UV) laser damage to fused silica, were observed for the first time in coatings of 532 nm polarizers. This suggests that, the polishing-induced subsurface damage of fused silica substrates may tend to be excited by a short-wavelength laser rather than the fundamental wavelength.

Temperature and Thermal Diffusivity Diagnostics in Laminar Methane Flames Using Infrared Four-Wave Mixing Techniques.

Four-wave mixing techniques, such as coherent anti-Stokes Raman spectroscopy (CARS), laser-induced grating spectroscopy (LIGS), and degenerate four-wave mixing (DFWM), have been widely used in combustion diagnostics due to their advantages of high signal-to-noise ratio (S/N), coherent signal, and spatial resolution. In this work, a nano-second pulsed laser is utilized to generate mid-infrared (near 3 µm) pump beams, exciting the rovibrational transitions of nascent water in flames. Combined LIGS and DFWM measurements are demonstrated in premixed laminar CH4/O2/N2 flames with equivalence ratios from 0.6 to 1.5, to achieve precise thermometry in a wide range of flame conditions. The flame temperatures were also measured by thermocouple as a reference, and the results from LIGS and DFWM align well with the trends shown in the thermocouple measurements. In fuel-lean flames, where the mass-to-specific-heat ratio variation is minimal, LIGS provides temperature data with a precision better than 16 K (0.8%). In fuel-rich flames, where the increased H2 concentration in the flame introduces uncertainty in gas constants thus affecting the accuracy of LIGS thermometry, DFWM is instead employed for temperature measurement since it is less sensitive to the gas composition within the measured volume. The high-precision LIGS temperatures in lean flames serve as temperature reference during the DFWM calibration of the degree of saturation, and a precision better than 90 K (4.5%) is achieved for DFWM thermometry. In addition to temperature, a theoretical model is employed to fit LIGS signal time waveforms, extracting the local speed of sound and thermal diffusivity with precisions better than 0.5% and 1.3%, respectively. These high-precision measurements contribute additional data for flame research and simulation calculations. This way, the combined use of DFWM and LIGS proves the potential for accurate thermometry and diagnostics of other thermodynamic parameters across a wide range of flame conditions.

Evaluation of the chemical states and electrical activation of ultra-highly B-doped Si1-xGex by ion implantation and subsequent nanosecond laser annealing

Given that transistordimensions are approaching atomic scale in metal–oxide–semiconductor field-effect-transistors (MOSFETs), attaining low-contact resistivity (ρc) between contact-metal and Si is a primary challenge for source/drain (S/D) fabrications. To reduce ρc, it is necessary to increase active dopant concentration in the semiconductor region underneath the contact-metal. High-dose ion-implantation and nanosecond laser-annealing (NLA) have been intensively investigated to produce highly activated layers in S/D regions. Particularly for the B-doped SiGe layers, which are used as S/D stressor for p-MOSFETs, non-substitutional B species are generated in the highly doped films at concentrations exceeding ∼ 1021 cm−3. These non-substitutional B are electrically non-activated and thus should be minimized to reduce ρc. In this study, we investigated the chemical states and electrical activation in highly B-doped SiGe layer. Using X-ray photoelectron spectroscopy (XPS), we examined the substitutionality of B-atoms and identified new chemical states of Si-B and Ge-B bonds in Si 2p and Ge 3d peaks. In the B 1s spectra, electrical activation states are determined by quantifying relative area ratios of the active/inactive B peaks, which are well matched with activation rates calculated by Hall measurements. Our findings systematically explained the activation behavior of NLA-treated B-doped SiGe films in high B-concentration ranges.

Effect of process parameters on the corrosion kinetics and mechanism of nanosecond laser surface structured titanium alloy (Ti6Al4V)

In this study, laser surface structuring (LSS) of Ti6Al4V was carried out using an Nd:YLF laser with a second harmonic wavelength of 527 nm and a pulse duration of 100 ns at varied laser fluence, scan speed, and line spacing. A detailed study of the effect of process parameters on surface topography, microstructure, composition, phase, contact angle, electrochemical behavior, and bioactivity was undertaken. Nanosecond pulsed laser irradiation with overlapping resulted in the formation of linear continuous grooves on the surface due to ablation/evaporation of materials. There is the formation of oxides of titanium (TiO2 and Ti2O3) whose mass fractions varied with process parameters. The average microhardness of the laser-structured region was improved (393 VHN - 535 VHN) as compared to the as-received Ti6Al4V (303 VHN). The contact angle of simulated body fluid (SBF) against the structured surface (58°- 123°) showed increased contact angle as compared to as-received samples (50°). The laser surface structuring exhibited a superior corrosion resistance property (in SBF) as compared to as-received Ti6Al4V. The mechanism of corrosion behavior was established by electrochemical impedance spectroscopic study (in SBF). The optimum process parameter for the LSS of Ti6Al4V with enhanced hardness and corrosion resistance was derived. The LSS surface processed under the optimum parameters measured by immersing in SBF revealed a higher deposition of calcium phosphate as compared to the as-received Ti6Al4V.

Comprehensive enhancement of mechanical properties of TZM/graphite joints via nanosecond laser fabricated striped structure

In this study, the bonding of a TZM alloy/graphite-brazed joint was improved by fabricating a striped structure on a graphite surface, effectively increasing the surface area ratio and surface roughness of graphite. The typical interfacial microstructure of the joint was TZM/(Ti, Mo)ss/TiNi/TiC/graphite. The shear strength of the joint increased and then decreased with an increase in laser scan distance, reaching a maximum value of 18.1 MPa at a laser scan distance of 90 μm. FEM simulations indicated that the formation of the striped structure optimised the residual stress distribution in the joint, with the stress state at the interface transformed from unilateral tensile stress to alternating tensile-compressive stress. Hence, the formation of the striped structure increased the joining area and optimised the stress distribution of the joints, thereby improving the mechanical properties of the TZM/ graphite-brazed joints.

Novel Approach to Color‐Neutral Transparent Solar Cells: An Organic‐Si Hybrid Achieved by Laser Microhole Drilling

AbstractTransparent solar cells (TSCs) hold promise for applications in building‐integrated photovoltaics (BIPVs) and vehicles. However, existing TSCs often have compromised color neutrality and efficiency because of the distinctive colors arising from their photoactive materials. The study proposes a novel fabrication method for TSCs based on Si wafers with the average visible transmittance (AVT) controlled by creating a microhole pattern through a combination of nanosecond laser drilling and subsequent wet etching. Through this ambient‐air process, organic–Si hybrid TSCs are successfully realized by integrating an organic PEDOT:PSS layer onto this structure with a neutral color. The power conversion efficiency (PCE) is adjusted by the fraction of the perforated region, and it attained 6.20% at 30% AVT, 5.37% at 40% AVT, and 4.67% at 50% AVT. In this hybrid TSC approach, the CIE color coordinates are closely aligned with the neutral‐color points (0, 0), resulting in color rendering index (CRI) values exceeding 99. The strategy represents a significant step toward the development of efficient color‐neutral TSCs for transparent solar applications.

Sub-ppb detection limit of Cr, Pb, and Cu in water by surface-enhanced LIBS with femtosecond laser

Water pollution caused by heavy metal contamination significantly threatens human health. Detecting trace levels of heavy metals in water is of utmost importance. Surface-enhanced laser-induced breakdown spectroscopy (SE-LIBS) has become a reliable detection method. In this study, we improved the analytical sensitivity of trace heavy metals in water by introducing femtosecond lasers into the SE-LIBS technique, replacing the traditional nanosecond lasers. An aluminum plate was utilized as a substrate to transform the liquid into a solid, overcoming the inadequacy in sensitivity and precision of LIBS when applied to water samples. We developed individual calibration curves for Cr, Pb, and Cu to determine their respective limit of detection (LOD). The LODs obtained were 0.3, 2.5, and 0.08 ng/mL for Cr, Pb, and Cu, respectively. These LODs were lower than traditional nanosecond SE-LIBS, and even lower than the LODs obtained by combining other LIBS enhancement techniques and SE-LIBS. We also determined good accuracy through the coefficient of determination R2, which was 0.97, 0.99, and 0.97, respectively, for Cr, Pb, and Cu. These results provide valuable and insightful information for the sensitive analysis of trace heavy metals in water using SE-LIBS with a femtosecond laser.