Transmission effects of high energy nanosecond lasers in laser-induced air plasma under different pressures

When a high energy nanosecond(ns)laser induces breakdown in the air,the plasma density generated in the rarefied atmosphere is much smaller than that at normal pressure.It is associated with a relatively lower absorption coefficient and reduces energy loss of the laser beam at low pressure.In this paper,the general transmission characterizations of a Joule level 10 ns 1064 nm focused laser beam are investigated both theoretically and experimentally under different pressures.The evolution of the electron density(n_(e)),the changes in electron temperature(T_(e))and the variation of laser intensity(I)are employed for numerical analyses in the simulation model.For experiments,four optical image transfer systems with focal length(f)of 200 mm are placed in a chamber and employed to focus the laser beam and produce plasmas at the focus.The results suggest that the transmittance increases obviously with the decreasing pressure and the plasma channels on the transmission path can be observed by the self-illumination.The simulation results agree well with the experimental data.The numerical model presents that the maximum n_e at the focus can reach 10^(19)cm^(-3),which is far below the critical density(n_(c)).As a result,the laser beam is not completely shielded by the plasmas.

A Comparison in Laser Precision Drilling of Stainless Steel 304 with Nanosecond and Picosecond Laser Pulses

Precision drilling with picosecond laser has been advocated to significantly improve the quality of micro-holes with reduced recast layer thickness and almost no heat affected zone.However,a detailed comparison between nanosecond and picosecond laser drilling techniques has rarely been reported in previous research.In the present study,a series of micro-holes are manufactured on stainless steel 304 using a nanosecond and a picosecond laser drilling system,respectively.The quality of the micro-holes,e.g.,recast layer,micro-crack,circularity,and conicity,etc,is evaluated by employing an optical microscope,an optical interferometer,and a scanning electron microscope.Additionally,the micro-structure of the samples between the edges of the micro-holes and the parent material is compared following etching treatment.The researching results show that a great amount of spattering material accumulated at the entrance ends of the nanosecond laser drilled micro-holes.The formation of a recast layer with a thickness of;5μm is detected on the side walls,associated with initiation of micro-cracks.Tapering phenomenon is also observed and the circularity of the micro-holes is rather poor.With regard to the micro-holes drilled by picosecond laser,the entrance ends,the exit ends,and the side walls are quite smooth without accumulation of spattering material,formation of recast layer and micro-cracks.The circularity of the micro-holes is fairly good without observation of tapering phenomenon.Furthermore,there is no obvious difference as for the micro-structure between the edges of the micro-holes and the parent material.This study proposes a picosecond laser helical drilling technique which can be used for effective manufacturing of high quality micro-holes.

A comparison of single shot nanosecond and femtosecond polarization-resolved laser-induced breakdown spectroscopy of Al

Aluminum samples have been analyzed by femtosecond polarization-resolved laser-induced breakdown spectroscopy （fs-PRLIBS）. We compare the obtained spectra with those obtained from nanosecond PRLIBS （ns-PRLIBS）. The main specific features of fs-PRLIBS are that a lower plasma temperature leads to a low level of continuum and no species are detected from the ambient gas. Furthermore, signals obtained by fs-PRLIBS show a higher stability than those of ns-PRLIBS. However, more elements are detected in the ns-PRLIBS spectra.

High-quality micro-shape fabrication of monocrystalline diamond by nanosecond pulsed laser and acid cleaning

The flat plane of small surface roughness below 0.1μm average roughness was obtained for monocrystalline diamond by nanosecond pulsed laser irradiation of 1060 nm and post-process acid cleaning,at a laser fluence around the material removal threshold value.The glossy and flat plane at the bottom of the micro-groove was parallel to the top surface of the specimen,although the round beam of Gaussian mode was irradiated in the direction perpendicular to the top surface of specimen.The square beam of top-hat mode produced a shallower micro-groove with a wider,flatter bottom compared with the round beam in Gaussian mode.The creation method of the flat plane with small surface roughness was discussed in the arrangement strategy of linear micro-grooving by the square beam of top-hat mode.Normal side-by-side repetition of linear micro-grooving did not create a flat plane with constant depth.Therefore,a two-step scanning method was proposed in order to overcome the problem in the normal side-by-side repetition of liner micro-grooving.Non-removal areas were partly retained between the processing lines in the first step,and the laser scanning was conducted on the retained area in the second step.The newly proposed two-step scanning method was practical and useful to create a widely flat plane with small surface roughness,and the two-step scanning method provided superior control over the micro-groove depth.This proposed method can reduce the surface roughness in addition to the shape creation of monocrystalline diamond,and it can be used as a high-quality micro-shape fabrication method of monocrystalline diamond.

pH Dependence of Photochemical Kinetics of Thioxanthen-9-one from Nanosecond Time-resolved Laser Flash Photolysis

pH dependent uorescence emission of a thioxanthone-based probe has been re-ported recently.The potential deter-minant factors of pH dependence may provide important clues to design novel thioxanthone-based probes in the fu-ture.pH dependence of photochemical kinetics of thioxanthone itself was inves-tigated in this work using nanosecond time-resolved laser ash photolysis.The nanosecond time-resolved transient absorption spectra and kinetics of TX in aqueous acetonitrile were recorded,as well as for a model reaction system including TX with diphenylamine(DPA)as a co-initiator.Besides the well-known absorption peak of ^(3)TX*,other peaks at 417,518,673 and 780 nm,have been reliably attributed to major intermediates in the overall reaction between TX and DPA with photolysis,which has been con rmed to occur along a multistep process.In the strong acidic solution(pH≈3.0),TX and protonated TX ions(TXH^(+))coexist due to protonated equilibrium.Consequently,high proton concentration promotes the predominant decay pathway after photolysis from electron transfer to proton affnity.Subsequently,the di erent primary products,^(3)TXH^(+*)or TX^(·-),proceed di erent secondary reaction channels.In addition,within the wide pH range from weak acid(pH=5.0)to alkaline solution(pH=13.0),the overall reaction mechanism and rates do not show visible changes.

Comparison of sample temperature effect on femtosecond and nanosecond laser-induced breakdown spectroscopy

In this paper,we investigated the emission spectra of plasmas produced from femtosecond and nanosecond laser ablations at different target temperatures in air.A brass was selected as ablated target of the experiment.The results indicated that spectral emission intensity and plasma temperature showed similar trend for femtosecond and nanosecond lasers,and the two parameters were improved by increasing the sample temperature in both cases.Moreover,the temperature of nanosecond laser-excited plasma was higher compared with that of femtosecond laser-excited plasma,and the increase of the plasma temperature in the case of nanosecond laser was more evident.In addition,there was a significant difference in electron density between femtosecond and nanosecond laser-induced plasmas.The electron density for femtosecond laser decreased with increasing the target temperature,while for nanosecond laser,the electron density was almost unchanged at different sample temperatures.

Time-resolved K-shell x-ray spectra of nanosecond laser-produced titanium tracer in gold plasmas

A study of a nanosecond laser irradiation on the titanium-layer-buried gold planar target is presented. The timeresolved x-ray emission spectra of titanium tracer are measured by a streaked crystal spectrometer. By comparing the simulated spectra obtained by using the FLYCHK code with the measured titanium spectra, the temporal plasma states, i.e.,the electron temperatures and densities, are deduced. To evaluate the feasibility of using the method for the characterization of Au plasma states, the deduced plasma states from the measured titanium spectra are compared with the Multi-1D hydrodynamic simulations of laser-produced Au plasmas. By comparing the measured and simulated results, an overall agreement for the electron temperatures is found, whereas there are deviations in the electron densities. The experiment–theory discrepancy may suggest that the plasma state could not be well reproduced by the Multi-1D hydrodynamic simulation, in which the radial gradient is not taken into account. Further investigations on the spectral characterization and hydrodynamic simulations of the plasma states are needed. All the measured and FLYCHK simulated spectra are given in this paper as datasets. The datasets are openly available at http://www.doi.org/10.57760/sciencedb.j00113.00032.

2-μm mode-locked nanosecond fiber laser based on MoS_2 saturable absorber

We demonstrated a 2-μm passively mode-locked nanosecond fiber laser based on a MoS2 saturable absorber（SA）.Owing to the effect of nonlinear absorption in the MoS2 SA, the pulse width decreased from 64.7 to 13.8 ns with increasing pump power from 1.10 to 1.45 W. The use of a narrow-bandwidth fiber Bragg grating resulted in a central wavelength and 3-dB spectral bandwidth of 2010.16 and 0.15 nm, respectively. Experimental results show that MoS2 is a promising material for a 2-μm mode-locked fiber laser.

Theoretical fundamentals of short pulse laser–metal interaction: A review

Short and ultrashort pulse lasers offer excellent advantages in laser precision machining mainly because of their high pulse energy and low ablation threshold. The complex process of laser interaction with metals limits the indepth investigation into laser ablation. Numerical simulation is important in the study of fundamental mechanisms. This review explores the start-of-the-art methods for the theoretical simulation of the laser ablation of metals, including plasma formation and expansion. Laser-induced period surface structures are also studied.

Numerical analyses on optical limiting performances of chloroindium phthalocyanines with different substituent positions

Optical limiting properties of two soluble chloroindium phthalocyanines with a- and β-alkoxyl substituents in nanosecond laser field have been studied by solving numerically the coupled singlet-triplet rate equation together with the paraxial wave field equation under the Crank-Nicholson scheme. Both transverse and longitudinal effects of the laser field on photophysical properties of the compounds are considered. Effective transfer time between the ground state and the lowest triplet state is defined in reformulated rate equations to characterize dynamics of singlet-triplet state population transfer. It is found that both phthalocyanines exhibit good nonlinear optical absorption abilities, while the compound with a-substituent shows enhanced optical limiting performance. Our ab-initio calculations reveal that the phthalocyanine with a-substituent has more obvious electron delocalization and lower frontier orbital transfer energies, which are responsible for its preferable photophysical properties.

The ablation characteristics of laser-assisted pulsed plasma thruster with metal propellant

In this study,a laser-assisted pulsed plasma thruster(LA-PPT)with a novel configuration is proposed as an electric propulsion thruster which separates laser ablation and electromagnetic acceleration.Owing to the unique structure of the thruster,metals can also be used as propellants,and a higher specific impulse is expected.The ablation quality,morphology,and plume distribution of various metals(aluminium alloy,red copper,and carbon steel)with different laser energies were studied experimentally.The ablation morphology and plume distribution of red copper were more uniform,as compared to those of other metals,and the ablation quality was higher,indicating its greater suitability for LA-PPT.The plume generated by nanosecond laser ablation of aluminium alloy expanded faster,which indicated that the response time of the thruster with aluminium alloy as the propellant was shorter.In addition,when the background pressure was 0.005 Pa,an obvious plume splitting phenomenon was observed in the ablation plume of the pulsed laser irradiating aluminium alloy,which may significantly reduce the utilisation rate of the propellant.

Oxidation mechanism of high-volume fraction SiCp/Al composite under laser irradiation and subsequent machining

Conventional mechanical machining of a composite material comprising an aluminum matrix reinforced with a high volume fraction of SiC particles(hereinafter referred to as an SiCp/Al composite)faces problems such as rapid tool wear,high specific cutting force,and poor surface integrity.Instead,a promising method for solving these problems is laser-induced oxidation-assisted milling(LOAM):under laser irradiation,the local workpiece material reacts with oxygen,thus forming loose and porous oxides that are easily removed.In the present work,the oxidation mechanism of SiCp/Al irradiated by a nanosecond pulsed laser is studied to better understand the laser-induced oxidation behavior and control the characteristics of the oxides,with laser irradiation experiments performed on a 65%SiCp/Al composite with various laser parameters and auxiliary gases(oxygen,nitrogen,and argon).With increasing laser pulse energy density,both the ablated groove depth and the width of the heat-affected zone increase.When oxygen is used as the auxiliary gas,an oxide layer composed of SiO_(2)and Al2O3 forms,and CO_(2)is produced and escapes from the material,thereby forming pores in the oxides.However,when nitrogen or argon is used as the auxiliary gas,a recast layer is produced that is relatively difficult to remove.Under laser irradiation,the sputtered material reacts with oxygen to form oxides on both sides of the ablated groove,and as the laser scanning path advances,the produced oxides accumulate to form an oxide layer.LOAM and conventional milling are compared using the same milling parameters,and LOAM is found to be better for reduced milling force and tool wear and improved machined surface quality.

Wettability regulation from superhydrophilic to superhydrophobic via nanosecond laser ablation

Metal surfaces play a crucial role in numerous applications,from self-cleaning and anti-icing to anti-fogging and oil-water separation.The regulation of their wettability is essential to enhance their performance in these areas.This paper proposes a multi-state regulation method for metal surface wettability,leveraging nanosecond laser ablation.By creating non-uniform microstructures on a metal surface,the contact area between the solid and liquid phases can be increased,resulting in the attainment of superhydrophilic properties(contact angle(CA),ranging from 4.6°to 8.5°).Conversely,the construction of uniform microstructures leads to a decreased solid-liquid contact area,thereby rendering the metal surface hydrophilic(CA=12.2°–53°).Furthermore,through heat treatment on a surface with uniform microstructures,organic matter adsorption can be promoted while simultaneously reducing surface energy.This process results in the metal surface acquiring hydrophobic properties(CA=92.1°–133.5°),facilitated by the“air cushion effect.”Building on the hydrophobic surface,stearic acid modification can further reduce surface energy,ultimately bestowing the metal surface with superhydrophobic properties(CA=150.1°–152.7°,and sliding angle=3.8°).Performance testing has validated the durability and self-cleaning effectiveness of the fabricated superhydrophobic surface while also highlighting the excellent anti-fog performance of the superhydrophilic surface.These findings strongly indicate the immense potential of these surfaces in various engineering applications.

Wetting of Laser Textured Cu Surface by Ethylene Glycol and Sn

The effect of microcosmic morphologies of textured Cu surface by nanosecond laser on the inert wetting and reactive wetting,i.e.,ethylene glycol/copper and tin/copper wetting systems,was studied by using modified sessile drop methods.To create different surface roughness,the microcosmic morphologies with different spacing of grooves were constructed by nanosecond laser.The results showed that the inert wetting(ethylene glycol/copper)was consistent with Wenzel model,while the reactive wetting results deviated from the model.In Sn/Cu reactive wetting system,the interfacial evolution in the early stage and the pinning of triple line by the precipitated η-Cu_(6)Sn_(5) caused the rougher surface and the worse final wettability.When the scale of artificial roughness exceeded the roughness that was caused by interfacial reaction after reaching the quasi-equilibrium state at interface,the final wettability could be improved.

Regulation of cell locomotion by nanosecond-laser-induced hydroxyapatite patterning

Hydroxyapatite,an essential mineral in human bones composed mainly of calcium and phosphorus,is widely used to coat bone graft and implant surfaces for enhanced biocompatibility and bone formation.For a strong implant-bone bond,the bone-forming cells must not only adhere to the implant surface but also move to the surface requiring bone formation.However,strong adhesion tends to inhibit cell migration on the surface of hydroxyapatite.Herein,a cell migration highway pattern that can promote cell migration was prepared using a nanosecond laser on hydroxyapatite coating.The developed surface promoted bone-forming cell movement compared with the unpatterned hydroxyapatite surface,and the cell adhesion and movement speed could be controlled by adjusting the pattern width.Live-cell microscopy,cell tracking,and serum protein analysis revealed the fundamental principle of this phenomenon.These findings are applicable to hydroxyapatite-coated biomaterials and can be implemented easily by laser patterning without complicated processes.The cell migration highway can promote and control cell movement while maintaining the existing advantages of hydroxyapatite coatings.Furthermore,it can be applied to the surface treatment of not only implant materials directly bonded to bone but also various implanted biomaterials implanted that require cell movement control.

Nanosecond laser conditioning of multilayer dielectric gratings for picosecond–petawatt laser systems

Multilayer dielectric gratings(MLDGs)are crucial for pulse compression in picosecond-petawatt laser systems.Bulged nodular defects,embedded in coating stacks during multilayer deposition,influence the lithographic process and performance of the final MLDG products.In this study,the integration of nanosecond laser conditioning(NLC)into different manufacturing stages of MLDGs was proposed for the first time on multilayer dielectric films(MLDFs)and final grating products to improve laser-induced damage performance.The results suggest that the remaining nodular ejection pits introduced by the two protocols exhibit a high nanosecond laser damage resistance,which remains stable when the irradiated laser fluence is more than twice the nanosecond-laser-induced damage threshold(nanosecond-LIDT)of the unconditioned MLDGs.Furthermore,the picosecond-LIDT of the nodular ej ection pit conditioned on the MLDFs was approximately 40%higher than that of the nodular defects,and the loss of the grating structure surrounding the nodular defects was avoided.Therefore,NLC is an effective strategy for improving the laser damage resistance of MLDGs.

Metal surface wettability modification by nanosecond laser surface texturing:A review

Laser surface texturing(LST)is a non-contact manufacturing process for fabricating functional surfaces in a manner that improves the corresponding wettability,and is widely used in biomedicine and industry.Laser surface texturing is a facile approach that is compatible with various materials,can result in a hierarchical texture,and enables a high degree of surface wetting(i.e.,extreme wetting).In addition to surface structures,surface chemical modification is a primary factor in producing extreme wetting surfaces.This review discusses the effects of various surface textures and surface chemistries on wettability.Optimal laser parameters for the desired surface texture are based on the fundamental wettability and laser mechanism.In particular,bumps in the morphology are conducive to obtaining extreme wetting.Diverse surface chemical strategies result in extreme wetting by different mechanisms.This paper makes a rigorous evaluation of the laser parameters and optimal surface chemical modifications by elucidating the relationships between the surface structure,surface chemical modification,and wettability,and in so doing,determines the final wettability.The unresolved problems of LST are presented in the conclusion.This review provides guidance,development directions,and an integrated framework for LST,which will be useful for fabricating extreme wetting surfaces on various metals.

516 mW,nanosecond Nd:LuAG laser Q-switched by gold nanorods

Q-switched operation of an Nd：LuAG laser using gold nanorods（GNRs） as the saturable absorber（SA） is reported, which also produces the highest average power among the nanosecond Nd-doped Q-switched lasers by GNRs-based SA. The applied GNRs are prepared using a seed-mediated growth method and then dropped onto the quartz substrate to fabricate the SA. The average power of the Q-switched laser is 516 mW with the shortest pulse duration of 606.7 ns and the repetition rate of 265.1 kHz.

Comparative study of amorphous and crystalline Zr-based alloys in response to nanosecond pulse laser ablation

In this work,we comprehensively investigate the response of amorphous and crystalline Zr-based alloys under nanosecond pulse laser ablation.The in situ multiphysics processes and ablation morphologies of the two alloy targets are explored and compared.The results indicate that the dynamics of laser-induced plasma and shock waves obey the idea blast wave theory and are insensitive to the topological structures of targets.Both targets experience significant superheating and culminate in explosive boiling.This ablation process leads to the formation of a hierarchical structure in the resultant ablation crater:microdents covered by widespread nanovoids.The amorphous target shows shallower microdents and smaller nanovoids than their crystalline counterparts because the former has a smaller heat-affected zone and experiences a higher degree of superheating.The hierarchical structure can adjust the surface wettability of targets from initial hydrophilic to hydrophobic,showing an increase of the contact angle approximately 119% for amorphous alloy compared with the crystal approximately 64%.This work demonstrates that amorphous alloys have a better performance against nanosecond pulse laser ablation and provides a feasible and one-step method of wettability modification for either amorphous or crystalline alloys.

Surface Micro-Nano Structures on GaN Thin Films Induced by 355 nm Nanosecond Laser Irradiation

Gallium nitride(GaN)has widespread applications in the semiconductor industry because of its desirable optoelectronic properties.The fabrication of surface structures on GaN thin films can effectively modify their optical and electrical properties,providing additional degrees of freedom for controlling GaN-based devices.Compared with lithography-based techniques,laser processing is maskless and much more efficient.This paper shows how surface micronano structures can be produced on GaN thin films using 355 nm nanosecond laser irradiation.The effects of the laser pulse energy,number of pulses,and polarization direction were studied.It was found that distinct micro-nano structures were formed under different irradiation conditions,and their geometries and elemental compositions were analyzed.The results indicate that different types of surface micro-nano structures can be produced on GaN thin films in a controllable manner using 355 nm nanosecond laser irradiation.The results of our study provide valuable guidance for the surface modification of GaN-based optoelectronic devices.