数值模拟飞秒激光加热金属的热电子发射

研究了超短超强激光脉冲与薄膜靶相互作用中产生的电子热发射.当超短激光脉冲与薄膜靶相互作用时,首先入射超短脉冲激光对吸收深度内的自由电子进行热激发,接下来热激发电子将能量传递到附近的晶格,再通过电子和晶格二体系的热传导,以及电子晶格间的热耦合,将能量传递到材料的内部.因此,电子在皮秒级甚至更短的时间内不能与晶格进行能量耦合,使电子温度超出晶格温度很多,电子热发射就变得非常明显了.用双温方程联合Richardson-Dushman方程的方法对飞秒脉冲激光照射金属靶的电子热发射进行了研究,结果发现电子热发射对飞     

飞秒激光烧蚀金属靶的冲击温度

在明确飞秒激光与物质相互作用过程冲击温度概念的基础上, 讨论了飞秒激光烧蚀铝靶和铜靶过程中的冲击温度与其他物理量的关系, 利用飞秒激光烧蚀金属的双温模型提取了冲击温度的绝对值, 基于非傅里叶热传导模型计算了冲击温度的分布. 此项研究结果对飞秒激光安全加工含能材料有借鉴意义. 关键词： 飞秒激光 含能材料 烧蚀 冲击温度     

短脉冲激光加工双温方程的优化运算

为研究双温方程中电子热传导项和电子与晶格耦合项对激光辐照物体表面温度场求解的影响,对这两项施加了约束条件。由于飞秒和亚皮秒激光与物质相互作用时间短,电子与晶格来不及耦合,所以对耦合项施加时间约束;根据相分离条件(CPPS),对热传导项施加空间约束。利用有限元方法建立了激光烧蚀金属铜膜表面的有限元模型。通过分析双温方程中热传导项和耦合项对计算结果的影响,发现短脉冲激光与铜金属相互作用过程中电子与晶格耦合项可以忽略,而传导项不可忽略。求解适当激光功率下的双温方程,得到了激光作用中心电子与晶格在不同脉冲宽度激光辐照下的温度变化关系。根据100 fs激光作用后晶格温度场的空间分布情况,研究了激光作用的相分离区域、相爆炸区域以及熔融区域的分布情况。     

Measurement of the electronic Grüneisen constant using femtosecond electron diffraction

We report the first accurate measurement of the electronic Grüneisen constant gamma(e) using a novel method employing the new technique of femtosecond electron diffraction. The contributions of the conduction electrons and the lattice to thermal expansion are differentiated in the time domain through transiently heating the electronic temperature well above that of the lattice with femtosecond optical pulses. By directly probing the associated thermal expansion dynamics in real time using femtosecond electron diffraction, we are able to separate the contributions of hot electrons from that of lattice heating, and make an accurate measurement of gamma(e) of aluminum at room temperature. This new approach opens the possibility of distinguishing electronic from magnetic contributions to thermal expansion in magnetic materials at low temperature.     

超短激光蚀除金属机制的分子动力学研究

 采用耦合电子热传导方程的分子动力学方法,研究了飞秒激光辐照下金属Ni的熔化及蚀除动力学。分析了靶材内部温度分布特征及蚀除产物的构成,主要包含单个原子及大团簇。确定了断裂位置和蚀除开始的标志,即该处温度分布出现小的峰值,且粒子数密度急剧下降。模拟结果表明：强烈的蒸发及靶材内部所产生的拉应力分别是单个原子及大团簇喷射的机制。同时,深入探讨了激光诱导压力波的传播规律,预测了压力波的波速,约为4.97 km/s。将不同脉冲能量密度下的蚀除速率同实验数据加以对比,结果相差16%~20%。预测了熔深随时间的变化规律,基本随时间的延续而呈上升的趋势。发现过热现象的存在。     

热物性参量对飞秒激光烧蚀金属影响的分子动力学模拟

利用结合双温模型的分子动力学模拟方法,研究了飞秒激光与金属相互作用的烧蚀机制.采用中心波长为800 nm,能量密度从0.043 J·cm~(-2)到0.40 J·cm~(-2)不等,脉宽分别为70 fs和200 fs的激光烧蚀金属镍和铝材料.靶材的温度、原子位型以及内部压力随时间的演化展示了材料热物性参量特性和激光参量对烧蚀结果的影响.结果显示材料电子热传导率对飞秒脉宽激光下的影响仍然较大;对比铝和镍的结果可知,铝的电子晶格耦合系数比镍的小,故电子晶格间的温度梯度持续时间较长;铝的电子热传导系数比镍的大,所以材料上下表面电子温度耦合的时间缩短.铝薄膜表面在能量密度为0.40 J·cm~(-2)激光烧蚀下呈现纳米尺寸的晶体结构.     

基于双相延迟模型的飞秒激光烧蚀金属模型

为了分析飞秒激光烧蚀过程,在双相延迟模型的基础上建立了双曲型热传导模型.模型中考虑了靶材的加热、蒸发和相爆炸,还考虑了等离子体羽流的形成和膨胀及其与入射激光的相互作用,以及光学和热物性参数随温度的变化.研究结果表明:等离子体屏蔽对飞秒激光烧蚀过程有重要的影响,特别是在激光能量密度较高时;两个延迟时间的比值对飞秒激光烧蚀过程中靶材的温度特性和烧蚀深度有较大的影响;飞秒激光烧蚀机制主要以相爆炸为主.飞秒激光烧蚀的热影响区域较小,而且热影响区域的大小受激光能量密度的影响较小.计算结果与文献中实验结果的对比表明基于双相延迟模型的飞秒激光烧蚀模型能有效对飞秒激光烧蚀过程进行模拟.     

发射光谱法对早期激光诱导铝合金等离子体诊断研究

对激光诱导等离子体参数进行诊断有多种方法,其中采用发射光谱法对其诊断是一种重要的方法。文中采用Nd∶YAG固态激光器,输出波长1 064 nm红外激光与铝合金样品相互作用,深入研究了铝合金等离子体产生早期(<1 μs)谱线轮廓、谱线强度、线背比、谱线半峰宽及位移等随时间演变规律。研究表明,激光与物质相互作用早期,电子数密度非常大,电子与离子及原子之间的相互作用非常强烈,谱线的Stark展宽效应非常明显,导致多重谱线重叠在一起,随时间演变,电子数密度及电子温度的降低,多重谱线的半峰宽越来越窄且谱线轮廓对称性越来越好。MgⅠ285.212 6 nm谱线强度早期逐渐增强,大约100 ns左右谱线强度达到最大值,然后谱线强度呈逐渐减小的趋势,这是由于等离子体产生早期,电子及离子占主导地位,故早期原子谱线强度较弱,随时间演变,电子与离子之间的复合,原子数密度逐渐增加,故原子谱线逐渐增强,达到最大值之后,由于等离子体激发温度的降低,故谱线强度逐渐减弱。以NIST波长位置为参考,等离子体产生的早期谱线发生了红移,连续背景强度随时间演变呈幂函数形式急剧递减,与之相反,谱线的连续背景强度与谱线强度相比,连续背景衰减的速度更为迅速,故导致谱线信背比随时间演变呈增大趋势,本研究对等离子体早期这些现象从理论角度进行了深入探讨。     

飞秒双脉冲激光照射金属薄膜的热行为

通过双温方程对飞秒单脉冲与双脉冲照射金薄膜进行了计算模拟分析,得到了金靶的电子温度和晶格温度随着时间空间的变化。在同样激光能量密度下,单脉冲与双脉冲使得金膜温度的变化表明双脉冲使得更多的激光能量渗透到靶材内部,这些能量可以使得烧蚀深度更深,有利于提高激光烧蚀靶材的效率。计算结果显示随着激光能量密度的增加熔化面深度逐渐增加,单脉冲与双脉冲熔化面深度的变化明显不同。在激光能量密度高于损伤阈值附近,单脉冲的烧蚀深度大于双脉冲的烧蚀深度,随着激光能量密度增加,双脉冲的烧蚀深度将大于单脉冲的烧蚀深度。     

Theoretical and experimental study on femtosecond laser induced damage in CaF2 crystals

We report on the damage threshold in CaF₂ crystals induced by femtosecond laser at wavelengths of 800 nm and 400 nm, respectively. The dependences of ablation depths and ablation volumes on laser fluences are also presented. We investigate theoretically the coupling constants between phonon and conduction band electrons (CBE), and calculate the rates of CBE absorbing laser energy. A theoretical model including CBE production, laser energy deposition, and CBE diffusion is applied to study the damage mechanisms. Our results indicate that energy diffusion greatly influences damage threshold and ablation depth. PACS 72.70.Jv; 42.88.+h; 78.20.Dj     

Ultrafast solid–liquid–vapor phase change of a gold film induced by pico- to femtosecond lasers

Melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are studied numerically. The nonequilibrium heat transfer in electrons and lattice is described using a two-temperature model. The solid–liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. An iterative procedure based on energy balance and gas kinetics law to track the location of liquid–vapor interface is utilized to obtain the material removal by vaporization. The effect of surface heat loss by thermal radiation was discussed. The influences of laser fluence and duration on the evaporation process are studied. Results show that higher laser fluence and shorter laser pulse width lead to higher interfacial temperature, deeper melting and ablation depths.     

飞秒激光等离子体中电子热传导现象的数值模拟

采用相对论电磁粒子模拟程序研究了飞秒激光等离子体相互作用中产生的电流密度、电场和自生磁场的发展演化过程。介绍了电子的非局域热输运的基本特性以及激光加热过程中温度烧蚀前沿稠密等离子体子区的预热效应、临界面附近的限流效应,以及冕区的反扩散与限流效应,得到了经典Spitzer-Harm理论描述的电子热传导随自生磁场的演化情形。数值模拟表明:在线性强激光作用下,由于电子初始时刻的无规则热运动,在等离子体上激发电磁不稳定性,而不稳定性激发的强电磁场使电子束在非常短的距离内沉积能量,同时对在激光有质动力推开电子时形成的超热电子能量输运产生抑制作用。     

Lattice deformation from interaction with electrons heated by an ultrashort laser pulse

We propose a model describing the destruction of metals under ultrashort intense laser pulses when heated electrons affect the lattice through the direct electron-phonon interaction. The metal consists of hot electrons and a cool lattice. The lattice deformation is estimated immediately after the laser pulse up to the electron temperature relaxation time. The hot electrons are described with help of the Boltzmann and heat conduction equations. We use an equation of motion for the lattice displacements with the electron force included. Estimates of the lattice deformation show that the ablation regime can be achieved. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 195–199 (10 August 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Ultra-short laser ablation of dielectrics: Theoretical analysis of threshold damage fluence and ablation depth

A coupled theoretical model based on Fokker–Planck equation for ultra-short laser ablation of dielectrics is proposed. Multiphoton ionization and avalanche ionization are considered as the sources during the generation of free electrons. The impact of the electron distribution in thermodynamic nonequilibrium on relaxation time is taken into account. The calculation formula of ablation depth is deduced based on the law of energy conservation. Numerical calculations are performed for the femtosecond laser ablation of fused silica at 526 and 1053 nm. It shows that the threshold damage fluences and ablation depths resulted from the coupled model are in good agreement with the experimental results; while the damage thresholds resulted from the approximate model significantly differ from the experimental results for lasers of long pulse width. It is concluded that the coupled model can better describe the micro-process of ultra-short laser ablation of dielectrics.     

Metal absorptivity in femtosecond pulsed laser ablation

In this paper, the effect of the absorptivity of metal on femtosecond pulsed laser ablation is investigated. The formulas for the absorptivity depending on target temperature are derived from Maxwell Equations and the Lambert-Beer’s law. Based on this, a new two-temperature model is proposed to describe the femtosecond pulsed laser ablation with metal. Then, using Au as an example, a finite difference method is employed to simulate the space-dependent and time-dependent absorptivity and the target temperature. The temperature evolution of our model is compared with the result obtained form the heat conduction model taking the absorptivity as constant. It is shown that the absorptivity plays an important role in the femtosecond pulsed laser ablation. The results of this paper are helpful in choosing the best technical parameters in femtosecond pulsed laser ablation.      

Temperature dependences of the electron-phonon coupling, electron heat capacity and thermal conductivity in Ni under femtosecond laser irradiation

The electron temperature dependences of the electron-phonon coupling factor, electron heat capacity and thermal conductivity are investigated for Ni in a range of temperatures typically realized in femtosecond laser material processing applications, from room temperature up to temperatures of the order of 10⁴ K. The analysis is based on the electronic density of states obtained through the electronic structure calculations. Thermal excitation of d band electrons is found to result in a significant decrease in the strength of the electron-phonon coupling, as well as large deviations of the electron heat capacity and the electron thermal conductivity from the commonly used linear temperature dependences on the electron temperature. Results of the simulations performed with the two-temperature model demonstrate that the temperature dependence of the thermophysical parameters accounting for the thermal excitation of d band electrons leads to higher maximum lattice and electron temperatures achieved at the surface of an irradiated Ni target and brings the threshold fluences for surface melting closer to the experimentally measured values as compared to the predictions obtained with commonly used approximations of the thermophysical parameters.     

飞秒多脉冲激光烧蚀金属过程中的能量剩余现象

 提出了描述整个飞秒多脉冲激光烧蚀过程的物理模型,模型考虑了多脉冲烧蚀的新的特点,考虑靶材吸收率随温度的变化和蒸发效应,建立了激光烧蚀不同阶段的热传导方程,给出了相应的定解条件。以金靶材为例,利用有限差分法,求解了热传导动力学方程,分别给出单脉冲和多脉冲作用下相应的靶材电子和离子亚系统的温度演化图像,及多脉冲激光作用下能量剩余系数和脉冲个数的变化规律,发现理论曲线与相应的实验数据吻合较好。研究结果充分证明多脉冲激光烧蚀过程中剩余能量的存在。     

Optical properties in nonequilibrium phase transitions

An open question about the dynamical behavior of materials is how phase transition occurs in highly nonequilibrium systems. One important class of study is the excitation of a solid by an ultrafast, intense laser. The preferential heating of electrons by the laser field gives rise to initial states dominated by hot electrons in a cold lattice. Using a femtosecond laser pump-probe approach, we have followed the temporal evolution of the optical properties of such a system. The results show interesting correlation to nonthermal melting and lattice disordering processes. They also reveal a liquid-plasma transition when the lattice energy density reaches a critical value.     

Thermal characteristics of double-layer thin film target ablated by femtosecond laser pulses

Thermal characteristics of tightly-contacted copper--gold double-layer thin film target under ablation of femtosecond laser pulses are investigated by using a two-temperature theoretical model. Numerical simulation shows that electron heat flux varies significantly on the boundary of copper--gold film with different maximal electron temperature of 1.15×10³ K at 5 ps after ablating laser pulse in gold and copper films, which can reach a balance around 12.6 ps and 8.2 ps for a single and double pulse ablation, respectively, and in the meantime, the lattice temperature difference crossing the gold--copper interface is only about 0.04×10³ K at the same time scale. It is also found that electron--lattice heat relaxation time increases linearly with laser fluence in both single and double pulse ablation, and a sudden change of the relaxation time appears after the laser energy density exceeds the ablation threshold.     

飞秒激光诱导铝等离子体时间分辨光谱研究

在大气环境下利用中心波长800nm、脉宽为30fs的激光聚焦在铝靶上,测定了激光诱导铝等离子体中铝原子的时间分辨发射谱。在局部热平衡条件近似下,根据实验测定的谱线相对强度得到了等离子体的电子温度;研究了激光脉冲能量对等离子体电子温度的影响和等离子体电子温度的时间演化特性。同时,实验发现了394.4nm和396.1nm两条铝原子谱线存在较强的自吸收效应,实验结果表明随着激光脉冲能量的减少和延时的增加,自吸收现象逐渐消失。