【作者】 李莉；

【导师】 张端明；

【作者基本信息】 华中科技大学 ， 凝聚态物理， 2007， 博士

【摘要】 脉冲激光沉积技术(Pulsed Laser Deposition,简称PLD技术)是近年来一种发展极为迅速的薄膜制备技术。这项技术较其他薄膜制备技术有激光能量高、薄膜的化学成分比与靶材一致、沉积速率高、基片温度要求低等多种优点。PLD技术制备薄膜的过程可划分为三个阶段:激光烧蚀、等离子体膨胀、薄膜沉积。我们课题组在制备出高质量的KTN薄膜材料的同时,以极大的精力将重点放在PLD机理的研究。建立了能够统一描述三个阶段的自洽的动力学模型Zhang-Li模型。并且,对PLD技术的每个阶段的物理机制和物理图象都进行了深入的研究。本文的研究主要集中在激光烧蚀阶段的材料固体光学性质的变化,及其对靶材烧蚀的影响。在充分考虑靶材吸收率和吸收系数随温度的演化规律的基础上,进一步针对超短脉冲激光(皮秒级和飞秒级)所带来的热传导的新的物理图象和多脉冲的累积效应,深入地探讨了激光烧蚀新的热传导规律和特点,建立了新的物理模型,理论预言和相关实验符合很好,并且为今后的理论发展提供了良好的物理平台。具体的讲,主要做了以下三方面的工作:首先,详细研究了脉冲激光与物质相互作用过程中靶材的烧蚀特点,分别从能量守恒和经典的电磁理论两个角度出发,给出了靶材吸收率和吸收系数随靶材温度动态变化的理论公式,结合恰当的边界条件,用有限差分法和分析法两种方法讨论了吸收系数和吸收率随时间的演化规律对靶材温度和融化深度的影响。其次,分析了皮秒级脉冲激光与材料发生烧蚀的物理图象,在考虑热传导的非傅立叶效应(即靶材中热传导速度的有限性)的基础上,建立了带有热源项的非傅立叶导热模型,研究了在高频脉冲激光烧蚀制备薄膜时,靶材温度分布的演化规律。最后,针对飞秒级多脉冲激光的烧蚀特点,提出了描述多脉冲飞秒激光烧蚀的一个新的物理模型。模型特点是:考虑了靶材吸收率的变化和蒸发效应对靶材温度的影响,尤为重要的是首次将多脉冲激光看作以脉冲持续时间和两脉冲间隔时间为周期的连续过程,并将每个周期的激光烧蚀分为三个不同的阶段:脉冲作用阶段(离子温度和电子温度之差增大)、双温持续阶段和傅立叶热传导阶段。利用此模型研究了靶材温度随脉冲个数的演化关系,并且着重研究了多脉冲激光烧蚀过程中的能量剩余现象。通过以上研究,我们发现,1.当靶材在脉冲激光辐照下发生烧蚀时,靶材吸收率和吸收系数随靶材温度呈现动态变化。当入射激光输入能量密度为高斯分布时,在脉冲开始和结尾阶段,系统状态主要取决于脉冲激光的能量输入,考虑吸收率和不考虑吸收率两种情况下的理论结果与实验结果偏差不大;在脉冲激光较长的中间阶段,忽略靶材吸收率的变化对于最终的模拟结果有重要影响,导致结果与实验数据有较大差异。2.当靶材在皮秒级脉冲激光的辐照下发生烧蚀时,a非傅立叶热传导的影响表现在热传导有一个滞后时间(热传导弛豫时间),越深的位置,滞后时间越长。从靶材温度随时间的演化趋势来看,在小于一个脉冲的时间区间,非傅立叶模型下靶材熔融前的温度上升速度明显高于通常的傅立叶热传导情况。并且,随着弛豫时间的增加,靶材的非傅立叶效应越来越明显。b在激光辐射10-11秒后,热源项对靶材表面温度的影响越来越显著。比较无源项模型的结果,靶材表面的温度上升越来越快。3.当靶材在飞秒级脉冲激光的辐照下发生烧蚀时,a在单脉冲激光作用下,电子和晶格两个子系统温度统一之后,由于热传导和蒸发效应的存在,靶材温度将会下降;并且在其他条件不变的前提下,无论随着激光功率密度还是脉冲宽度的增加,靶材温度所需的耦合时间都会增大。b在多脉冲激光作用下,电声耦合时间之后的靶材温度随着脉冲个数依次上升,充分证明,在飞秒多脉冲激光作用下,若后表面绝热,材料内存在能量积累效应。更多还原

【Abstract】 Pulsed laser deposition method is a new thin-film preparation technology with many advantages, and it has been developing very quickly. The PLD technique has so many advantages, such as high pulsed laser energy density, similarity between target and prepared thin film components, the high deposition rate, and lower substrate temperature. The whole process can be divided into three stages: laser ablation, plasma expansion and film growth. Combined PLD technology with other techniques, high-quality KTN thin films are prepared by our group. Based on accumulated experimental experiences, our researches emphasis is laid on theoretical mechanism on PLD technology. Recently, a more integrated Zhang-Li (Z-L) model has been proposed. Every stage of PLD processes are considered as a uniform whole in this model. At the same time, the physics mechanism and image of every state of PLD processes is investigated detailed.The paper aims in the ablation process of PLD. Based on consulting other experiments, Zhang-Li model and local theoretical works of other researchers, physical phenomena in ablation stage are completely studied in detail, such as the target optical characteristic, and two new models are present to describe the heat conduction in ablation. Three works are fulfilled:Firstly, the physical picture of ablation stage is discussed. The dynamic absorptance and absorption coefficient as the function of irradiation time and incident laser intensity are derived from two points of view: law of conservation of energy, and Maxwell Equations with Lambert Beer’s law respectively. By finite difference and analytical method, the effect of dynamic absorptance and absorption coefficient on target temperature and melt depth is studied. The calculated results demonstrate that the effect of the dynamic absorptance and absorption coefficient on PLA is notable:Secondly, a non-Fourier conduction model with heat source term is presented to study the target temperature evolvement when the target is radiated by picosecond pulsed laser. By Laplace transform, the analytical expression of the space- and time-dependence of temperature is derived. The corresponding physical mechanism is analyzed.Lastly, a new theoretical model of multipulse femtosecond laser ablation has been developed to describe the behavior of target temperature. The characteristic of this model is that, the femtosecond laser ablation is considered as a periodic process,and the periods is the laser pulse and the interval between two pulse. Every pulse is divided into three different phases (pulse laser irradiation stage, two temperature continue stage, Fourier heat conduction stage). In this model, the temperature dependence of the absorptance and the vaporization is considered.The results of above investigation shows that1. As the target is ablated by pulse laser,as the Gaussian distribution is introduced to describe the incident laser intensity,the dynamic absorptance and absorption coefficient show little effect on PLA due to the small intensity at the beginning and the end of a pulse,while in the longer middle period of a pulse, the dynamic absorptance and absorption coefficient exhibit their effect on the state of the system due to the steady incident laser intensity. Their effect on the temperature after melting is larger than that before melting.2. As the target is ablated by picosecond pulsed laser,a. The effect of non-Fourier conduction behaves mainly that there is a delay of heat conduction in the inner target, and the deeper the target is, the longer the delay time is. The non-Fourier temperature is distinctly higher than that of the Fourier heat conduction model.b. The effect of heat source term becomes more and more remarkable after the target is radiated for 10-11 s in the condition of this paper. Compared to the results of the model without heat source term, the increase of surface temperature obtained by non-Fourier model is quicker.3. As the target is ablated by femosecond pulsed laser,a. For single pulse laser irradiation, the target temperature will decrease for the heat conduction and vaporization after the coupling time of electron and lattice. With the increase of either laser intensity or laser width, the coupling time becomes longer.b. For multipulse laser irradiation, the target temperature after coupling time ascends with the pulse number. This proves that energy residua indeed play an important role. And the descending of target temperature will be rapid more and more due to the evaporation.更多还原