脉冲激光烧蚀材料等离子体反冲压力物理模型研究与应用

分析了脉冲激光烧蚀材料等离子体等温膨胀阶段的物理特性,建立了脉冲激光烧蚀材料等离子体压力三维方程与动力学模型.应用所建模型,数值分析了单脉冲激光烧蚀青铜金刚石砂轮等离子体相关特性,得到等离子体的反冲压力最大值870 Pa出现在约25 ns后,距离砂轮表面距离约0.05 mm处.相关条件下开展脉冲激光烧蚀青铜金刚石砂轮试验,采用高速相机观测烧蚀砂轮过程中的飞溅现象;采用光栅光谱仪测量等离子体空间发射光谱,计算了等离子体电子温度、电子密度以及反冲压力.实验表明脉冲激光烧蚀青铜金刚石砂轮等离子体反冲压力可以不计,同时也验证了气体方程与动力学模型的正确性和可行性,对脉冲光纤激光烧蚀工艺优化具有启示意义.     

飞秒激光烧蚀硅表面产生等离子体的发射光谱研究

对中心波长为800 nm,脉宽为100 fs的激光脉冲烧蚀空气中硅(111)产生的等离子体发射光谱进行了时间和空间分辨研究. 结果表明,在等离子体羽膨胀初期(小于50 ns时间范围内),等离子体发射光谱主要由连续光谱构成,此后连续光谱强度逐渐减弱,线状光谱开始占主导地位;在羽体膨胀过程中离子谱线的存在时间短于原子谱线的存在时间. 由时间分辨发射光谱发现在羽体膨胀过程中等离子体辐射波长存在红移现象,波长红移量随时间演化呈二次指数衰减. 最后给出等离子体发射光谱谱线强度的时空演化规律. 关键词： 飞秒激光 脉冲激光烧蚀 等离子体 发射光谱     

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     

脉冲激光烧蚀中电声弛豫时间的确定

为了提高脉冲激光制备薄膜的质量,准确掌握电声弛豫时间是关键,它对脉冲激光脉宽和能量密度的选取起着决定性的作用. 文中以铝靶材为例,利用经典的双温方程通过时域有限差分法(FDTD)得到电子、离子亚系统的温度随时间和位置演化的图像,进而得到电声弛豫时间的准确值. 这样便能准确划分热烧蚀和非平衡烧蚀,从而更好地控制激光的烧蚀过程. 同时找出了电声弛豫时间随激光脉宽以及能量密度变化的规律. 关键词： 飞秒激光 电声弛豫时间 双温方程 激光能量密度     

脉冲激光制膜过程中等离子体演化规律的研究

利用有限差分法对脉冲激光沉积(PLD)技术制备KTa065Nb035O3(KTN)薄膜过程中等离子体在等温和绝热两个阶段的速度演化进行了模拟，并给出了其中主要粒子在空间的具体演化规律，对等离子体在空间膨胀的物理机制，进行了深入的讨论，给出了相应演化过程的物理图像，并揭示了等离子体羽辉在膨胀过程中呈现椭球外形的内在原因.     

飞秒激光诱导锗等离子体辐射光谱特性

由于飞秒激光脉冲宽度小于靶材电子—晶格热弛豫时间,飞秒激光烧蚀靶材过程以及诱导击穿产生的等离子体膨胀动力学过程与纳秒激光作用过程不同,因此研究飞秒激光诱导等离子体发射光谱特性对于研究飞秒激光烧蚀机制以及飞秒激光诱导等离子体的膨胀动力学过程非常重要。Ge材料是一种常用的中远红外探测器以及光学元器件材料,对中心波长为800 nm,脉宽为50 fs的激光脉冲烧蚀空气中Ge靶材产生的等离子体发射光谱强度的时间和空间演化规律研究,并探讨了飞秒激光脉冲能量对等离子体发射光谱强度的影响规律。实验结果表明在等离子体羽膨胀初期,飞秒激光诱导Ge等离子体发射光谱主要由线状光谱和连续光谱构成,在200 ns时间内连续光谱强度逐渐减弱,线状光谱开始占主导地位。通过探测Ge等离子体的时间分辨发射光谱,随着等离子体的快速膨胀,等离子体发射光谱强度随着时间的增加呈现先增加后下降变化,在335 ns达到最大。通过探测Ge等离子体的空间分辨发射光谱,随着距离Ge靶材表面的位置增加,等离子体发射光谱强度随远离Ge靶材表面距离增加呈现先增加后下降变化,在0.8 mm位置达到最大。由于存在等离子体自吸收机制,等离子体发射光谱...     

飞秒激光烧蚀石英玻璃的实验与理论研究

实验研究了800nm飞秒激光作用下石英玻璃的破坏机理和烧蚀规律，给出了破坏阈值与脉冲宽度的关系.发展了雪崩击穿模型，计算了材料的烧蚀阈值与脉冲宽度的依赖关系，烧蚀深度、烧蚀体积与脉冲能量的依赖关系，研究了导带电子的扩散对材料中激光能量的沉积、分布，以及材料的破坏阈值和烧蚀规律的影响. 关键词： 飞秒激光脉冲 破坏机理 石英玻璃 电子扩散     

KTN薄膜脉冲激光沉积过程的机理研究

根据能量平衡原理,导出了脉冲激光作用靶材的烧蚀率公式,并根据流体动力学理论,得出了脉冲激光产生的等离子体的空间运动特性方程,将靶材烧蚀率公式与等离子体空间动力学方程结合起来,根据实验研究了不同激光功率密度和波长对Kta_0.65Nb_0.35O₃(KTN)薄膜沉积特性的影响,得到了一些有价值的结果,并对结果进行了详细的讨论,理论计算结果与实验大体符合. 关键词： PLD技术 烧蚀率 等离子体 KTN薄膜     

短脉冲激光加工硅膜的传热过程及烧蚀图形的二维计算

为研究短脉冲激光辐照硅膜表面后的能量传输过程,基于双温方程的计算方法以及自由电子气理论,建立了求解能量传输方程的二维有限元模型.针对红外以及可见光波段的激光,通过限制硅膜的大小,有效地控制了计算的精度,并得到电子温度与热流的时间以及空间分布.计算结果表明,激光诱导产生的等离子体密度极大地影响了硅膜表面的反射率及光吸收系数;通过分析电子热流密度随时间的变化曲线,得到硅膜内部能量的传输过程;在激光作用过程中,硅膜内部晶格温度始终保持在熔点以下,证明了等离子体密度是激光烧蚀硅膜的主导因素;预测了激光烧蚀的图形,并分析了不同波长的激光烧蚀图形与高斯曲线的关系.     

飞秒激光脉冲与金属光阴极相互作用

本文从理论上分析了飞秒激光与光阴极相互作用过程，采用双温模型分析了飞秒激光脉冲辐照下金属薄膜的温度效应。通过建立一个简单的光电效应模型，获得了最佳的金属光阴极厚度，通过该模型可以发现，产生的光电流对在飞秒激光脉冲辐照下的电子温度和晶格温度有着很大的依赖关系。     

Modeling the triggering of streamers in air by ultrashort laserpulses

The physical processes involved in the triggering of ionization waves (streamers) by ultrashort laser pulses, focused in air at 350 Torr and in a uniform electric field, are investigated by means of a one-dimensional (1-D) numerical model. The model describes the interaction of the laser pulse with air and takes into account many of the reactions in the laser-created plasma as well as the radial expansion of the plasma. Consequences of the model are that the threshold electric field for the appearance of streamers is an increasing function of the delay between the laser pulse and the electric field pulse and a decreasing function of the laser energy. Also, it appears that the electron temperature, the plasma density and radius, and the conduction of heat across the plasma boundaries play major roles in the capacity of the laser-created plasma to trigger streamers. The results of the model are compared with the available experimental data     

In situ STM of pulsed laser nanostructured deposits: First stages of film formation

In the synthesis of nanostructured thin films the characterization of the growth processes plays a fundamental role for the control of the film and surface properties. Moreover when the deposition technique is based on the production and the assembling of nanoparticles/clusters the characterization of the precursor size distribution is of fundamental importance.We have designed a pulsed laser deposition (PLD) apparatus for the production of nanostructured thin films and surfaces, connected to a UHV variable temperature scanning tunneling microscope (STM). The whole system is devoted to the synthesis and in situ study of nanostructured and nanoporous functional metal and metal oxide films and surfaces. We have deposited W nanoparticles produced by a few hundreds laser pulses in order to investigate the initial mechanisms of the film growth. Different deposition conditions have been explored by controlling the laser generated plasma expansion through a background gas in the PLD chamber. STM measurements have been performed on W thin films deposited on different substrates to study both the size distribution and the aggregation of the precursors on the surface. Although substrate effects must be taken into account, the control of the background gas pressure and of the target-to-substrate distance allows to produce surfaces with different morphologies. This opens the possibility to tailor the material properties through the control of the size and deposition energy of the building nano-units.     

Ionization effect to plasma expansion study during nanosecond pulsed laser deposition

The dynamic characteristic and effects of plasma play an important role in film growth process of pulsed laser deposition (PLD). Based on numerical hydrodynamic modeling, supposed the laser radiation and partial ionization of the plasma as a dynamic source, we deduce a set of new plasma expansion dynamics equations. Based on which, as an example of carbon target, using finite difference method, the plasma flow dynamics evolvement in vacuum is investigated. Our results show the dynamic partial ionization increases the expansion in all directions, which changes into a new dynamic source for plasma expansion. In the axial direction, because of the collisional interactions between particles, the plume density peak is in the vicinity of the target surface and the acceleration of plasma occurs mainly near the target surface too. In the transverse direction, the plume peak is not near the target, but at the surface. The space expansion distance is far less than the axial direction because there is no high initial velocity component in this direction. The predictions of the plasma expansion action based on the proposed dynamics source assumption are found to be in agreement with the experimental observation.     

基于混合加宽的宽带激光脉冲放大的物理模型

建立了宽带激光脉冲放大的物理模型.模型引入交叉弛豫时间表征不同频率响应下反转粒子与激光作用后重新形成分布的快慢，模型考虑的整体非均匀加宽、局部均匀加宽的混合加宽机理弥补了现有放大模型仅考虑均匀加宽的不足.基于此物理模型，开发了宽带激光传输放大的模拟设计软件CPAP，并讨论了在一定条件下此宽带模型可回归到窄带模型.     

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.     

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.     

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.     

纳秒激光烧蚀冲量耦合数值模拟

为研究激光烧蚀靶产生冲量过程和机理, 建立了一个复杂的一维热传导和流体动力学模型. 以空间碎片常见材料Al为例, 用建立的模型数值计算了纳秒脉宽激光烧蚀靶产生的冲量及冲量耦合系数随时间变化情况. 数值结果和已有的实验数据符合的较好. 数值计算表明: 激光脉冲时间内, 靶获得的冲量随时间迅速增加, 在脉冲时间结束后, 冲量变化随时间趋于稳定; 在冲量耦合过程中, 烧蚀等离子体向真空膨胀, 羽流尺度逐渐增大, 同时吸收入射激光能量, 导致激光与靶耦合的能量降低. 关键词： 激光烧蚀 冲量耦合 等离子体     

Direct simulation monte carlo of pulsed laser ablation of metals with clusterization processes in vapor

A mathematical model is proposed to describe laser ablation of metals in vacuum under the action of nanosecond laser pulses of moderate intensity taking into account the processes of cluster formation and decay in the vapor cloud. To describe the laser radiation absorption and metal heating, the thermal model based on the unsteady one-dimensional heat equation with a volume heat source is used, and the method of statistical modelling is employed for modelling vapor expansion and the processes of cluster formation. The efficiency of the proposed complex model is considered by the example of pulsed laser ablation of a niobium target. The work was supported financially by INTAS (grant No. 03-51-5208).     

飞秒脉冲烧蚀单晶硅的超快动力学

利用基于Pump-probe系统的超快时间分辨阴影图的方法,研究了空气中飞秒激光烧蚀单晶硅的动力学过程。实验采用脉宽为50 fs、平均能量密度约35 J/cm2的单脉冲激光烧蚀单晶硅,获取飞秒激光烧蚀单晶硅过程中等离子体和冲击波的形成和发展过程的时间分辨阴影图。实验结果表明:飞秒激光烧蚀单晶硅导致其表面物质喷发的过程是不连续的,分为明显的两次喷发过程。这表明飞秒激光与单晶硅作用的过程中,在不同的时间段可能由不同的机制主导,在前期可能是多光子电离为主,在后期可能是由多光子效应和雪崩效应共同作用。研究还发现,延迟时间较长时,冲击波形状发生畸变。