Improved thermal model and its application in UV high-power pulsed laser ablation of metal target

An improved thermal model describing UV high-power nanosecond laser ablation of metal target is presented. The vaporization effect, the plasma shielding effect, as well as the absorption coefficient and absorptivity dependence of temperature are considered in this model. Take iron target as an example, the numerical solutions are obtained from the heat flow equations (before and after melting) using a finite difference method. The space and time dependence of temperature of the target, the time dependence of temperature at two different positions for a certain laser fluence, and the ablation rate as a function of laser fluence are also presented. The numerical results that agreed well with the experimental data are much better than the one without, which indicates that the above two effects and the temperature dependence of absorption coefficient and absorptivity in UV high-power laser ablation of metal target should not be neglected. We hope the present model will be useful for further experimental investigation of metal thin films prepared by pulsed laser deposition.     

A model of laser ablation with temperature-dependent material properties,vaporization, phase explosion and plasma shielding

Laser ablation of metals using nanosecond pulses occurs mainly due to vaporization. However, at high fluences, when the target is heated close to its critical temperature, phase explosion also occurs due to homogeneous nucleation. Due to a wide variation in target temperature, the material properties also show a considerable variation. In this paper, a model of laser ablation is presented that considers vaporization and phase explosion as mechanisms of material removal and also accounts for the variation in material properties up to critical temperature using some general and empirical theories. In addition, plasma shielding due to inverse bremsstrahlung and photo-ionization is considered. The model predicts accurately (within 5 %) the phase explosion threshold fluence of Al. The predictions of ablation depth by the model are in reasonable agreement with experimental measurements at low fluences. Whereas, the degree of error marginally increases at high laser fluences.     

激光诱导等离子体的气体动力学和燃烧波扩展速度研究

针对激光对熔石英材料产生致燃损伤过程中存在的激光支持燃烧波,考虑激光作用的温度残余、目标形貌的改变、喷溅物质分布、目标表面气流状况的分布等效应,分阶段对激光支持燃烧波的过程进行建模和仿真研究.通过建立二维轴对称气体动力学模型,模拟研究包含逆韧致辐射、热辐射、热传导和对流过程在内的激光能量传输过程.此外,依据激光支持燃烧波在可见光波段具有明显的辐射特征这一特点,利用阴影法测量了激光对熔石英致燃损伤过程中的燃烧波扩展速度,得到了燃烧波演化过程图像.研究结果表明:在平行激光束作用下,燃烧波的传播是稳态的,气体动力学行为比较稳定;在聚焦激光束作用下,燃烧波的传播是非稳态的.模拟结果中得到的激光支持燃烧波扩展速度及气体动力学结构与实验结果和理论推导结果符合得很好,验证了理论模型的正确性.     

A New Synthetical Model of High-Power Pulsed Laser Ablation

We develop a new synthetical model of high-power pulsed laser ablation, which considers the dynamic absorptance, vaporization, and plasma shielding. And the corresponding heat conduction equations with the initial and boundary conditions are given. The numerical solutions are obtained under the reasonable technical parameter conditions by taking YBa₂Cu₃O₇ target for example. The space-dependence and time-dependence of temperature in target at a certain laser fluence are presented, then, the transmitted intensity through plasma plume, space-dependence of temperature and ablation rate for different laser fluences are significantly analyzed. As a result, the satisfactorily good agreement between our numerical results and experimental results indicates that the influences of the dynamic absorptance, vaporization, and plasma shielding cannot be neglected. Taking all the three mechanisms above simultaneously into account for the first time, we cause the present model to be more practical.     

A New Synthetical Model of High-Power Pulsed Laser Ablation

We develop a new synthetical model of high-power pulsed laser ablation, which considers the dynamic absorptance, vaporization, and plasma shielding. And the corresponding heat conduction equations with the initial and boundary conditions are given. The numerical solutions are obtained under the reasonable technical parameter conditions by taking YBa2CusO7 target for example. The space-dependence and time-dependence of temperature in target at a certain laser fluence are presented, then, the transmitted intensity through plasma plume, space-dependence of temperature and ablation rate for different laser fluences are significantly analyzed. As a result, the satisfactorily good agreement between our numerical results and experimental results indicates that the influences of the dynamic absorptance, vaporization, and plasma shielding cannot be neglected. Taking all the three mechanisms above simultaneously into account for the first time, we cause the present model to be more practical.     

用计算机数据采集系统测量液氮的汽化潜热

在液氮汽化潜热的测量实验中,通过计算机数据采集系统准确记录了液氮汽化过程中质量的变化,并由铜的定压比热随温度变化的曲线准确计算出了液氮的汽化潜热.     

Single-walled carbon nanotube formation with double laser vaporization technique

Single-walled carbon nanotubes (SWNTs) were prepared with double laser vaporization of a graphite target and a metal/alloy target inside an electric furnace at 1200 ^°C ambient temperature with 500 torr Ar gas atmosphere. Each target was vaporized simultaneously with a different Nd:YAG laser. Several kinds of metal/alloy target (Ni, Co, Fe, and permalloy) were tested in order to see the difference in the resulting SWNT yield and the diameter distribution of them. The Raman spectra of SWNT-containing soot prepared by use of this technique with permalloy/carbon system indicated that permalloy gives almost the same yield as compared with Ni/Co carbon composite rod with single laser vaporization technique, though the diameter distribution of them is slightly different. Also, time-resolved images of the plume by carbon and permalloy nanoparticles after laser vaporization were collected using a high-speed video camera. These images suggest that the hot plumes due to carbon and permalloy nanoparticles do not mix together so extensively, at least in a few hundred microseconds after laser vaporization. The effect of time delay between two laser pulses on the yield and the diameter distribution of SWNTs was also presented and discussed.     

Vaporization and Plasma Shielding during High Power Nanosecond Laser Ablation of Silicon and Nickel

A thermal model to describe the high-power nanosecond pulsed laser ablation is presented. It involves the vaporization and the following plasma shielding effect on the whole ablation process. As an example of Si target, we obtainthe time evolution of the calculated surface temperature, ablation rate and ablation depth. It can be seen that plasma shielding plays a more important role in the ablation process with time. At the same time, the ablation depth with laser fluence based on different models is shown. Moreover, we simulate the pulsed laser irradiation Ni target. The evolution of the transmitted intensity and the variation of ablation depth per pulse with laser fluence are performed. Under the same experimental conditions, the numerical results calculated with our thermal model are more in agreement with the experimental data.     

纳秒脉冲激光沉积薄膜过程中的烧蚀特性研究

研究了高能短脉冲激光薄膜制备的整个烧蚀过程.首先建立了基于超热理论的烧蚀模型，然 后利用较为符合实际的高斯分布表示脉冲激光输入能量密度，给出了考虑蒸发效应不同阶段 的烧蚀状态方程.结合适当的边界条件，以Si靶材为例，利用有限差分法得到了靶材在各个 阶段温度随时间和烧蚀深度的演化分布规律及表面蒸发速度与烧蚀深度在不同激光辐照强度 下随时间的演化规律.结果表明，在脉冲激光辐照阶段，靶材表面的蒸发效应使得靶材表面 温度上升显著放缓；在激光辐照强度接近相爆炸能量阈值时，蒸发速度与蒸发厚度的变化由 于逆流现象将显著放缓.还得到了考虑了熔融弛豫时间及蒸发效应的固-液界面随时间的演化 方程，这一结论较先前工作更具有普适性. 关键词： 脉冲激光烧蚀 热流方程 温度演化 有限差分法     

Pulsed laser-induced ablation of absorbing liquids and acoustic-transient generation

2 CrO₄ are irradiated by a KrF excimer laser (λ=248 nm, FWHM=24 ns) with moderate energy density (up to 100 MW/cm²) below the plasma-formation threshold. The ablation process, including the vapor-cavity formation and the acoustic-wave propagation is visualized by laser-flash photography. The ablation thresholds are determined by measuring the generated pressure transients and vapor-phase kinetics using a broadband piezoelectric pressure transducer and a simultaneous optical-transmission probe, respectively. The mechanisms of liquid ablation and acoustic-pulse generation are investigated based on the thermoelastic behavior of the liquid medium and the evaporation dynamics. A numerical model is proposed to describe the explosive-vaporization process at high laser fluences. The computation results are compared with the experiment. In short-pulse heating, ablation can be initiated at low laser fluences by the tensile component of the thermoelastic stress without a significant increase in the liquid temperature. On the other hand, if the heating rate is rapid enough to achieve a high degree of superheating of the liquid, the abrupt increase of the homogeneous-bubble-nucleation rate leads to explosive vaporization, which then plays the major role in the ablation dynamics. The pressure transient in the liquid is generated thermoelastically at low laser fluences, but the contribution of the vapor-phase expansion and/or the recoil momentum exerted by the ablation plume becomes significant at high laser fluences. Shock waves are formed in the ambient air in the case of explosive vaporization. The propagation of these wave fronts is in good agreement with the numerical-computation results. Received: 8 February 1998/Accepted: 10 February 1998     

Picosecond laser ablation of nano-sized WTi thin film

Interaction of an Nd:YAG laser, operating at 532 nm wavelength and pulse duration of 40 ps, with tungsten-titanium (WTi) thin film (thickness, 190 nm) deposited on single silicon (100) substrate was studied. Laser fluences of 10.5 and 13.4 J/cm² were found to be sufficient for modification of the WTi/silicon target system. The energy absorbed from the Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following WTi/silicon surface morphological changes were observed: (i) ablation of the thin film during the first laser pulse. The boundary of damage area was relatively sharp after action of one pulse whereas it was quite diffuse after irradiation with more than 10 pulses; (ii) appearance of some nano-structures (e.g., nano-ripples) in the irradiated region; (iii) appearance of the micro-cracking. The process of the laser interaction with WTi/silicon target was accompanied by formation of plasma.     

液氘状态方程实验数据测量

在神光Ⅱ高功率激光装置上建立了液氘状态方程实验研究系统, 在80 min内实现控温范围12–300 K可调、控温精度±0.03 K、机械震动 ≤20 μm的实验控制精度; 通过镀膜窗口质量筛选和靶体清洁工作解决了低温下窗口材料残余反射率高的难题, 获得了信噪比较好的实验图像; 利用神光II第九路输出3ω (351 nm)、3 ns、1000 J的能力, 采用阻抗匹配方法, 配合任意反射面速度干涉仪诊断系统, 在国内首次获得液氘在约60 GPa压力下的冲击绝热线实验数据, 数据与国外同压力区间数据符合较好, 为下阶段约100 GPa压力范围液氘状态方程的实验研究奠定了基础.     

超声波雾化辅助液体样品激光诱导击穿光谱技术研究

受到液体内部复杂环境因素的影响,LIBS技术在针对液体样品的成分检测分析的应用受到很大的制约.文章提出了一种由超声波雾化器件辅助,先将液体雾化成空气中密集的雾状小液滴,然后进行LIBS检测的新方法.并依据此思路实现了一套由超声波雾化器、Nd∶YAG激光器(1 064 nm)和ICCD探测器组成的检测系统.使用该系统对溶解有镁元素的水溶液样品进行了系统实验分析.实验发现,即使在相对较低的脉冲能量(30 mJ)下,该方法诱导的LIBS信号仍具有较长的寿命和较高的信背比.该方法对Mg元素的检测限能够达到0.242 ppm(10-6 g·g-1)量级.同时,文章还利用Ha线对等离子体的电子密度进行了计算,分析了等离子体演化的特点.     

Modeling of laser induced breakdown spectroscopy for very low-pressure conditions

Laser Induced Breakdown Spectroscopy (LIBS) can be considered as a prominent technology for compositional analysis of materials in low-pressure space applications. In space applications, usually LIBS is conducted in a low-pressure environment and proper understanding of the plasma parameters is significant for any improvement in the system. A model is developed to describe the heating and subsequent melting, vaporization and ionization of a target material during LIBS process. A numerical model based on one-dimensional thermal conductivity equation is being used to simulate the target evaporation and a hydrodynamic model is used to simulate plume expansion. Further, an experimental approach of measuring spectral emission from the ablation plume using emission spectroscopy and estimating the plasma state, such as the ionization species, and average plasma temperature, is investigated. An important result of this work is that for different ambient conditions, laser ablation plume dynamics can be estimated.     

Thermal model for nanosecond laser sputtering at high fluences

The vaporization effect and the following plasma shielding generated by high-power nanosecond pulsed laser ablation are studied in detail based on the heat flux equation. As an example of Si target, we obtain the time evolution of the calculated surface temperature, ablation rate and ablation depth by solving the heat flow equations using a finite difference method. It can be seen that plasma shielding plays a more important role in the ablation process with time. At the same time, the variation of ablation depth per pulse with laser fluence is performed. Our numerical results are more agreed with the experiment datum than other simulated results. The result shows that the plasma shielding is very important.     

激光脉冲对内层等离子体辐射源的影响

本文的基本思想是设计双层金铝薄膜靶以检测激光脉冲宽度与等离子体消融深度的关系,找出有效的等离子体加热方法以产生更强更亮的等离子体辐射源.由于有预脉冲激光的存在,表层金薄膜首先被消融,由主脉冲携带的大能量就能较易穿过表层金等离子体将能量聚焦在内层铝靶上,由此产生内层高温等离子体.又由于外层低温等离子体存在,其将有效的阻碍内高温等离子体因膨胀而引起的能量损失.对无预脉冲而言,直接入射激光能量都沉积在靶表层形成表层高温等离子体.但是激光直接入射而产生的等离子体辐射总强度只比由预脉冲情况下产生的金等离子体辐射强度增加15%.而预脉冲能量只占激光总能量的2%.实验结果显示Al光谱线主要来自类氢,类氦离子跃迁.Au等离子体光谱线主要来自它的N带,O带和P带谱.我们也观察到一个明显的软X射线短波发射极限.所有结果显示由于预脉冲的存在将对靶各层等离子体辐射产生极大的影响     

Investigation of pulsed laser ablation process of Hg0.8Cd0.2Te

The vaporization threshold was measured under the irradiation of 1.064-μm, 10-ns pulsed laser. Then we calculated the vaporization temperature based on the conservation law of energy and analyzed the vaporization time based on our established model. These results coincided well with the information from the micrograph of scanning electron microscope (SEM) and the spectra of the plasma. Besides, the laser ablation rate was also computed and discussed theoretically.     

Optical and microscopy investigations of soot structure alterations by laser-induced incandescence

2 at 1064 nm, vaporization/fragmentation of soot primary particles and aggregates occurs. Optical measurements are performed using a second laser pulse to probe the effects of these changes upon the LII signal. With the exception of very low fluences, the structural changes induced in the soot lead to a decreased LII intensity produced by the second laser pulse. These two-pulse experiments also show that these changes do not alter the LII signal on timescales less than 1 μs for fluences below the vaporization threshold. Received: 20 October 1997/Revised version: 16 February 1998