Some aspects of laser-metal vapour interaction

The vapour beam interaction plays an important role in laser machining process, since the vapour generated on the workpiece surface absorbs some fraction of the incident beam and heats the workpiece surface. Consequently, a study of this interaction mechanism is essential. For this purpose, a computer program was developed to investigate the interaction interaction, it provides a foundation to do further studies. It was found that the vapour locally high pressure gradients. Although the analysis presents a simplified picture of the interaction, it provides a foundation to do further studies. It was found that the vapour temperature reaches 5000 K after 10⁻⁶ s for a laser pulse of 10¹¹ W/m² power intensity. The leading edge of the vapour velocity had a velocity of the order of 4000–7000 m/s.     

激光等离子体相互作用的局域振荡电子加热机制

 用2.5维粒子模拟程序模拟了超强激光与等离子体的相互作用过程，发现超强激光可以通过J×B加热机制加速电子并引起电荷分离，从而产生很强的静电场并形成电场势阱，电子在静电场势阱中振荡，被多次加速，使得高速电子被甩出势阱，进而增强电荷分离，然后静电场结构被破坏，静电势能传给电子。在此过程中，电子在此阱中作局域振荡，并且被J×B机制多次加速，激光的能量会有效地传给电子，使电子能量高达10MeV。这是一种新的电子加热机制，称之为局域振荡电子加热机制。     

Measurement of laser beam transmittance through laser produced vapour plume

Laser produced plume, consisting of vapor front and ejected substrate on the workpiece surface, is an intermediate between the incident laser beam and the workpiece on which the beam is directed. It partially blocks, defocuses, absorbs, scatters, and deflects the incident beam and thereby reduces the laser energy reaching the workpiece. However, plume additionally acts as a heat source enhancing the machining. Consequently, laser induced plume plays an important role in laser machining process. The present study investigates the transmittance of a reference beam by the plume generated during the laser-workpiece interaction. To achieve the transmittance measurements, a Nd:YAG laser was used and four different materials were employed in the experiment. To obtain realistic values of the plume transmittance in relation to the laser machining process, the reference beam was sampled from the incident laser beam. The study was extended to include the effect of the laser pulse parameters on the transmission process. It is found that about 10–30% of the transmittance of the reference beam through a vapor plume, produced due to laser ablation, occurs close to the workpiece surface.     

激光等离子体相互作用的局域振荡电子加热机制

用2.5维粒子模拟程序模拟了超强激光与等离子体的相互作用过程,发现超强激光可以通过J×B加热机制加速电子并引起电荷分离,从而产生很强的静电场并形成电场势阱,电子在静电场势阱中振荡,被多次加速,使得高速电子被甩出势阱,进而增强电荷分离,然后静电场结构被破坏,静电势能传给电子。在此过程中,电子在此阱中作局域振荡,并且被J×B机制多次加速,激光的能量会有效地传给电子,使电子能量高达10MeV。这是一种新的电子加热机制,称之为局域振荡电子加热机制。     

Heating mechanism in relation to the laser machining process

Laser workpiece interaction mechanism is an important phenomenon which will assist in the development of laser machining systems. The interaction mechanism is generally complicated and depends on the laser and workpiece properties. In the present study a mathematical analysis for the laser material removal by evaporation and radial ejection of liquid is carried out. In the analysis the time unsteady problem is solved and nucleation explosions are predicted.     

激光等离子体相互作用的数值模拟

 基于激光等离子体相互作用的复杂物理过程的数学模型，采用PIC方法分别研究了P极化和S极化非均匀短脉冲强激光入射均匀分布的稠密等离子体时引起的空泡、成丝等物理现象。模拟了激光脉冲在真空中的3维传播形貌。由3维密度分布图发现：激光产生的巨大的有质动力向两侧推动粒子，形成等离子体密度通道；当激光脉冲入射等离子体区域后，纵向加速的电子速度峰值出现在电流峰值处。     

Study of magnesium and aluminum alloys absorption coefficient during Nd:YAG laser interaction

In laser processes, the absorption factor of laser Nd:YAG by metals plays a very important role. In order to model laser welding, we need to know its evolution during the process. The theoretical calculation does not enable the prediction of the absorption factor in the case of a keyhole mode. It is difficult to predict the effect of plasma and recoil pressure on the shape of the keyhole. In this paper, an integrating sphere is used to determine the absorption factor during the laser process, which is carried out on two types of magnesium alloys (WE43 and RZ5) and an aluminum alloy. We obtain the evolution in time of the absorption factor according to different steps of the evolution of the keyhole.     

高聚物中分子相互作用对红外吸收谱的影响

基于扩展SSH模型及动力学演化的方法,通过数值计算研究了链间耦合作用对高聚物吸收谱的影响.计算结果表明,随着链间相互作用的增大,会导致低频吸收边发生红移,这与实验结果一致.进而发现,对于呈电中性的体系,吸收边红移有两种不同的理论根源:第一种情况是带隙减小导致的红移;第二种情况是电子从带间态向导带电子态跃迁导致的吸收边红移.     

高聚物中分子相互作用对红外吸收谱的影响

基于扩展SSH模型及动力学演化的方法,通过数值计算研究了链间耦合作用对高聚物吸收谱的影响。计算结果表明,随着链间相互作用的增大,会导致低频吸收边发生红移,这与实验结果一致。进而发现,对于呈电中性的体系,吸收边红移有两种不同的理论根源：第一种情况是带隙减小导致的红移;第二种情况是电子从带间态向导带电子态跃迁导致的吸收边红移。     

一种含圆柱形谐振散射体的黏弹材料低频吸声机理研究

利用层多重散射法分析了一种含圆柱形谐振散射体黏弹 材料吸声层在钢背衬条件下的低频吸声特性. 该吸声层由包覆软材料的圆柱空腔周期嵌入橡胶材料中构成, 其中, 散射体轴线与吸声层平行. 结果表明, 20 mm厚吸声层在1000–3000 Hz具有良好的吸声性能. 通过综合分析单个散射体的吸收截面、单层周期散射体的声吸收、 结构内部位移场以及耗散功率密度分布, 揭示了该吸声层的低频吸声机理. 关键词： 水声吸声 吸收截面 耗散功率密度     

Investigation of the multiplet features of SrTiO3 in X‐ray absorption spectra based on configuration interaction calculations

Synchrotron‐based L_2,3‐edge absorption spectra show strong sensitivities to the local electronic structure and chemical environment. However, detailed physical information cannot be extracted easily without computational aids. Here, using the experimental Ti L_2,3‐edges absorption spectrum of SrTiO₃ as a fingerprint and considering full multiplet effects, calculations yield different energy parameters characterizing local ground state properties. The peak splitting and intensity ratios of the L₃ and L₂ set of peaks are carefully analyzed quantitatively, giving rise to a small hybridization energy around 1.2 eV, and the different hybridization energy values reported in the literature are further addressed. Finally, absorption spectra with different linearly polarized photons under various tetragonal crystal fields are investigated, revealing a non‐linear orbital–lattice interaction, and a theoretical guidance for material engineering of SrTiO₃‐based thin films and heterostructures is offered. Detailed analysis of spectrum shifts with different tetragonal crystal fields suggests that the e_g crystal field splitting is a necessary parameter for a thorough analysis of the spectra, even though it is not relevant for the ground state properties.     

Single-mode operation of an Ar+ laser with an intracavity I2 absorption cell

A multi-mode argon ion laser was furnished with an intracavity¹²⁷I₂ absorption cell for the purpose of mode suppression at the wavelength of 514.5 nm. Singlemode operation was achieved with an 8 cm long cell having a¹²⁷I₂ vapour pressure of 0.27 mbar (0.2 Torr).     

Comparative interaction mechanisms for different laser systems with selected materials on titanium alloys

This paper describes further research into, and modelling of, the interaction mechanisms of various laser types with materials, including synthetic oil and silicone grease. A Q-switched Nd:YAG pulsed laser, a CW CO₂ laser, and a pulsed KrF excimer laser (λ = 248 nm) were used in the study. In general, the materials studied were very absorptive in the ultraviolet, less absorptive in the infrared, and least absorptive in the near-infrared. For the excimer, photo-ablation takes place, while for the other two wavelengths thermal vapourization dominates. In the case of silicone grease, full volatilization is only obtained using the excimer. For the other two wavelengths, this is not the case, with a sticky residue remaining after treatment. Interaction with synthetic oil with its lower boiling point can be obtained for all three laser types. With the Nd:YAG the dominant heating mechanism is conduction from the substrate and a baking effect is observed with strong interaction between the contaminant and the substrate. For the two pulsed lasers, oxidation is substantially less, while for the continuous wave CO₂, the interaction time is sufficiently long for oxidation and secondary reactions to take place if care is not taken to work below the reaction threshold.     

Exciton states and optical absorption in quantum wires under laser radiation

We analyze the exciton states in a quantum wire under intense laser radiation. Electrons and holes are confined by the parabolic potential of the quantum wire. An exactly solvable model is introduced for calculating the exciton binding energy, replacing the actual Coulomb interaction between the electron and the hole by a projective operator.     

Final-state-interactions in the infrared absorption of doped semiconductors

We calculate the profile of the infrared inter-valence-band absorption in p-doped non-polar semiconductor as modified by the final-state interaction of electronic excitations with optical phonons. The absorption coefficient vanishes at a light frequency equal to the frequency of the non-polar optical vibrations. The spectrum is asymmetric with respect to the absorption minimum. We show that the type of asymmetry depends on the position of the optical phonon energy relative to the electronic continuum. The results are compared to available data for p-doped Si.     

相对论效应对激光在等离子体中的共振吸收的影响

采用一维粒子模拟(PIC)方法，研究了相对论效应对P偏振激光斜入射非均匀等离子体时产生的共振吸收的影响. 计算表明，弱相对论情况下，在临界面附近产生的电子等离子体波的相对论非线性效应占主要作用；随着入射光场的逐渐增大，吸收率逐渐降低. 当入射光强超过3.7×10¹⁷W/cm²时，由于超短激光脉冲本身在等离子体中产生相对论效应、等离子体波破裂效应，以及参量不稳定过程激发等，吸收系数随着激光强度又开始增加. 固定等离子体密度标长，取不同的激光入射角、电子初始温度，相对论效应对吸收系数的影响是一致的. 关键词： 激光等离子体 相对论效应 共振吸收 粒子模拟     

Y型Langmuir-Blodgett膜中分子聚集行为的偶极作用模型

利用经典静电场理论，描述了Y型Langmuir-Blodgett (LB) 膜中棒状分子聚集行为的偶极作用模型，给出了Y型LB膜结构与光谱性质的关系.讨论了Y型膜中分子间距、层间距、分子取向、膜层数等结构参数对分子聚集行为的影响，并在此基础上比较了Y型膜和Z型膜分子聚集行为的异同.理论结果与实验规律符合较好.     

Segregation during crystal growth from melt and absorption cross section determination by optical absorption method

Segregation during crystal growth from melt under two conditions is studied by using crystal mass, which can be measured easily, as an independent variable, and a method to determine the effective segregation coefficient and absorption cross section of optical dopant is given. When the segregated solute disperses into the whole or just a part of melt homogenously, the concentration C _s in solid interface will change by different formulas. If the crystal growth interface is conical and segregated solute disperses into melt in total or part, the solute concentration at r = 2/3R, where r is the distance from the growth cross section center and R the crystal radius, is independent on the shape of the crystal growth interface, and its variation at r = 2/3R can be regarded as the result from crystal growth in flat interface. With C _s variation formula in solid and absorption cross section σ for optical dopant, the absorption coefficients along the crystal growth direction can be calculated, and the corresponding experimental value can be obtained through the crystal optical absorption spectra. By minimizing the half sum, whose independent variables are k, ΔW or σ, of the difference square between the calculated and experimental absorption coefficients from one or more absorption peaks along the crystal growth direction, k and σ, or k and ΔW, can be determined at the same time through the Levenberg-Marquardt iteration method. Finally, the effective segregation coefficient k, ΔW and absorption cross sections of Nd:GGG were determined, the results fitted by two formula gave more closed effective segregation coefficient, and the value ΔW also indicates that the segregated dopant had nearly dispersed into the whole melt. Experimental results show that the method to determine effective segregation coefficient k, ΔW and absorption cross sections σ is convenient and reliable, and the two segregation formulas can describe the segregation during the crystal growth from melt relatively commendably. Supported by the National Natural Science Foundation of China (Grant No. 50772112)     

Microstructure and interface interaction in laser induction hybrid cladding of Ni-based coating

Based on the powder feeding laser induction hybrid cladding experiments by means of three different laser and induction energy, the microstructure of Ni-based coating and interface characteristics between the Ni-based coating and steel substrate were investigated. The results show that the hybrid cladding energy including laser and induction energy has an important influence on the formation of the interface and the microstructure of the Ni-based coating characterized by the dendrite. In addition, the laser and induction energy can complement each other. For high hybrid cladding energy, the single phase Fe-Ni-Cr solid solution is formed at the interface between the coating and substrate, while the microhardness of the Ni-based coating decreases. For low hybrid cladding energy, the solid solution phases of Fe-Ni-Cr and Ni-Fe-Cr are respectively obtained on both sides of the interface and microhardness of the Ni-based coating is relatively high. During laser induction hybrid cladding, the metallurgical bond characterized by the white light layer is achieved between the coating and substrate, and the extent of metallurgical reaction can be controlled by adjusting the laser energy and induction energy appropriately.     

The role of surface absorption coefficient in the thermal field of the laser–thin film interaction

In the present paper we developed a model of laser–thin film interaction, in order to find the thermal fields during laser irradiation. The approximation consists in the fact that k (the thermal conductivity) and γ (the thermal diffusivity) are of the same order of magnitude for the substrate and thin film. In consequence we can consider that we have one solid sample with k and γ of the Si substrate. On the other hand we cannot say the same thing about . One can easily see that, _SiCu,Al,Ag,_Au. In consequence we propose an absorption coefficient, as close as possible to reality. In the present paper, we then calculate the thermal fields produced by the surface absorption.