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.

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.

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.

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.

All-fiber, high-average-power nanosecond laser based on core-diameter adjustment

Core-diameter adjustment,in analogy to doping management,is proposed in this letter for balancing thermal load and nonlinear efects.In this scheme,the core-to-cladding ratio increases with increasing core diameter along the direction of signal and pump propagation.An all-fber-integrated Yb-doped master oscillator power amplifcation(MOPA) is successfully demonstrated.Two segments of fber with diferent core diameters but same inner-cladding diameters and doping levels are spliced together and used as gain fbers in the last stage.A maximum average output power of 200 W at an overall slope efciency of 71% is achieved from the MOPA with a pulse energy of 2 mJ and peak power as high as 80 kW.

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.

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.

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^25μm 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.

Formation and Growth of Silver Nanocubes upon Nanosecond Pulsed Laser Irradiation: Effects of Laser Intensity and Irradiation Time

Silver nanoparticles (AgNPs) were fabricated by repetitive irradiation of near ultraviolet (UV) nanosecond laser pulses (355 nm, 5 ns) in an aqueous solution of silver nitrate in the absence of stabilizing agents. A broad absorption peak was observed in the visible region showing the formation of a variety of AgNPs in the solution. Among the variety of products, it was found that silver nanocubes (AgNCs) grew in size with longer laser irradiation time. The size of AgNCs also increased with higher laser intensity. The average size of AgNCs, investigated by a scanning electron microscope (SEM) was in the range of 75 - 200 nm. The number of reduced atoms in AgNCs as a function of laser intensity showed that the AgNCs are apparently produced by a four photon process, implying that the formation of dimer silver atoms is essential for the formation.

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.

PHOTOPHYSICS STUDIES OF ORGANIC COMPOUNDS——NANOSECOND LASER PHOTOLYSIS AND TRANSIENT ABSORPTION SPECTRA STUDIES OF PYRENE-AROMATIC AMINES

Nanosecond time-resolved transient absorption spectra of pyrene-triphenylamine, pyrenediphenylamine, pyrene-N, N-dibenzylaniline systems in various solvents have been investigated. In nonpolar solvent, pyrene-triphenylamine exciplex and pyrene-N, N-dibenzylaniline exciplex were observed directly. In acetonitrile, the pyrene anion radical, triphenylamine and diphenylamine cation radicals were detected. On the basis of the present experimental facts, the mechanism of fluorescence quenching processes of pyrene in polar and nonpolar solvents has been established.

Measurement of K, L intermediate lifetime of purple membrane excited by nanosecond laser pulse

Purple membrane, molecular weight 26ku, is synthesized by Holobacterium halobium on some parts of its cell membrane under a special condition combined by 248 residues,and 7 times acrossing membrane. Only one kind of protein, bacteriorhodopsin (BR), is on the membrane. After light excitation, BR goes through a series of intermediate processes K, L, M, N, O and recovers to BR. Purple membrane is an ideal biomaterial for photo energy transfer, and becomes a new focus field of many branches of science. Because the photocycle of BR is related to light-driven proton pump and photoelectric response,

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.

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 MoS_2 saturable absorber(SA).Owing to the effect of nonlinear absorption in the MoS_2 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 MoS_2 is a promising material for a 2-μm mode-locked fiber laser.

LEO航天器太阳电池阵激光诱导电弧放电试验与分析

基于太阳电池阵空间电弧放电理论,对纳秒脉冲激光诱导太阳电池阵电弧放电机理进行分析,采用空间放电模拟试验系统,试验研究了低地球轨道(LEO)等离子体环境下纳秒脉冲激光诱导太阳电池阵电弧放电的功率密度阈值及其放电特性。试验结果表明:LEO等离子体环境下激光诱导太阳电池阵电弧放电的功率密度阈值约为1.08×10^(8)W/cm^(2),与无激光诱导相比,放电持续时间增加了20%,且每施加1次纳秒脉冲激光诱导电弧放电概率约为60%。对比分析不同工况下的太阳电池阵损伤效应可得:纳秒脉冲激光诱导电弧放电破坏绝缘层最严重,容易造成太阳电池阵中部分PN结短路失效,其损坏程度远远大于空间环境太阳电池串间电弧放电烧蚀损坏效应和盖玻片表面热应力损伤造成的PN结断路失效。此结论可为航天器太阳电池阵放电试验的措施及试验参数选取提供参考。

无保护层纳秒脉冲激光强化对DD5单晶合金耐磨性能的影响

目的提高发动机涡轮叶片榫头材料DD5单晶高温合金的高温耐磨性能。方法采用不同能量的无保护层纳秒脉冲激光对DD5单晶合金试样表面进行处理,利用摩擦磨损试验机获得DD5单晶合金在650℃下的摩擦磨损性能参数,并通过显微硬度计、扫描电子显微镜、能谱仪、共聚焦三维表面轮廓仪对磨损前后的DD5单晶试件进行表征分析,揭示纳秒脉冲激光强化技术提高单晶高温合金耐磨性能的机理。结果采用纳秒脉冲激光处理DD5单晶高温合金后,材料表层发生了重熔现象,并在重熔层中生成了弥散分布的铝的氧化物颗粒,其粒径为5~50nm;纳秒脉冲激光处理后产生的弥散分布的氧化铝颗粒为硬质相,它提升了单晶表面的显微硬度;经纳秒脉冲激光处理后,试样的磨损率降低了19.7%~37.6%,单晶合金的磨损机制以磨粒磨损为主。结论经纳秒脉冲激光处理后,单晶表面生成了含有弥散分布的纳米氧化铝颗粒的重熔层,在它与激光诱导的等离子体冲击波产生的硬化层的共同作用下,提升了镍基单晶高温合金在高温下的耐磨性能。

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.