【作者】 房然然；

【导师】 张端明；

【作者基本信息】 华中科技大学 ， 凝聚态物理， 2009， 博士

【摘要】 脉冲激光沉积（PLD，Pulsed Laser Deposition）技术的迅猛发展和诱人的应用前景，使其成为当今世界的研究热点之一，其在薄膜和纳米粒子制备技术方面的优势更是独树一帜。该领域的实验进展一直远超前于理论进展。目前，伴随着激光锁模技术和啁啾技术的实验发展，能量超高化和脉宽超短化成为脉冲激光向极端条件发展的两个主要方向，相应的，在高能与超短激光脉冲条件下的PLD实验技术的进一步发展，呼唤着理论研究的不断进步。本文比较系统和深入地研究了纳秒级和飞秒级PLD技术的脉冲激光烧蚀（PLA，Pulsed Laser Ablation）动力学，细致地讨论了纳秒级PLA过程中的蒸发效应、等离子屏蔽效应以及动态吸收效应；飞秒级PLA过程中的非傅里叶热传导效应、电子—电子碰撞、电子态密度改变引起的效应（即DOS效应：Density of States）等对于靶材烧蚀的影响。在纳秒量级内将整个热物理过程细致地分为熔融前和熔融后两个子过程，构建了更为合理的热传导动力学模型；在飞秒级PLA动力学研究中取得了更为丰硕的成果：构建了能统一描写从纳秒级到飞秒级的统一双温方程（TTM，Two Temperature Model），能反映电子和电子碰撞效应的改进TTM以及适用于更高能域（包括DOS效应）的新的TTM。在上述模型的框架内，我们分别以金属Ni，Au，Fe，Cu及高温超导YBa₂Cu₃O₇（YBCO）等靶材为对象，对其热传导性质进行了模拟研究，结果均与现有的文献更为精确。我们的研究结果，对于正确深入地了解PLA过程中的物理图像具有重要的理论价值。本文的结构组织如下：第一章简要综述了PLD技术发展及其动力学研究进展，尤其分析了纳秒级PLA和飞秒级PLA过程的特性，并将两者进行了对比。第二章和第三章介绍了我们对高能纳秒（紫外和红外）脉冲激光对单组分金属靶材和多组分氧化物钇钡铜氧超导体靶材YBCO的烧蚀特性研究的新成果。首先，从激光辐照结束后的超热效应和靶材烧蚀相变特性分析出发，将烧蚀过程分为靶材熔融前和熔融后两个子过程，给出了不同子过程下的热传导方程，细致地考虑了蒸发效应，等离子屏蔽效应以及动态吸收效应，构建了更为合理的热传导模型。尤其值得指出的是，我们提出了平均电离能的概念，成功地描述了相应的等离子屏蔽效应，解决了研究多组分靶材的等离子体屏蔽效应的瓶颈问题。在新的模型框架内，给出了靶材温度随时间和深度的演化分布规律。最后，详细讨论了蒸发效应，等离子屏蔽效应以及动态吸收效应对烧蚀过程的影响。第四章在深入分析纳秒级PLA的平衡烧蚀和飞秒级PLA的非平衡烧蚀的基础上，通过引入电声耦合时间（ΥR）和脉冲宽度（ΥL）的比值构建了平滑过渡参数，建立了能够描述从纳秒到皮秒、飞秒级脉冲激光烧蚀的热物理现象的非傅立叶统一双温方程TTM。利用此方程，我们细致地研究了电子和晶格亚系统随时间和位置的演化规律，以及蒸发阈值随脉宽的变化规律。第五章深入探讨了飞秒激光的脉宽和能量密度对电声驰豫时间的影响。研究发现，当激光能量密度固定时，脉宽越短，电声驰豫时间越短。当脉宽固定时，激光能量密度越低，电声驰豫时间越长。第六章和第七章系统地研究了随着能量的进一步提高，高能飞秒激光烧蚀机制发生深刻变化的情况。第六章针对电子温度大于4000K的情况，研究发现此时电子—电子碰撞效应必须考虑。在同时考虑电子—晶格碰撞和电子—电子碰撞的基础上，构建了改进的双温方程。利用改进的双温方程，研究了靶材表面的电子和晶格亚系统随时间的演化规律和靶材内的电子和晶格亚系统随时间和烧蚀深度的三维变化规律。第七章研究了电子温度超过10000K时，在烧蚀机制中必须进一步考虑DOS效应的变化。在此基础上，以过渡金属Ni和贵重金属Au为例，研究了由DOS效应引起的热物理参数随温度的变化规律，然后利用新的TTM对不同厚度薄膜的融化阈值进行了数值模拟，发现理论值与实验值符合的较好，该研究表明，当电子温度超过10000K时，DOS效应是不能被忽略的。本文主要有如下创新之处：（1）构建了考虑蒸发效应、多组分等离子屏蔽效应以及动态吸收效应的不同烧蚀阶段的热传导方程组，将方程组分为靶材熔融前、熔融后两个子过程的相应热传导方程。（2）首次建立了能统一描述从纳秒到皮秒、飞秒级脉冲激光烧蚀的热物理现象的非傅立叶双温方程，并进一步探讨了电子和晶格亚系统温度随时间和位置的演化规律，以及不同脉宽下的蒸发阈值。（3）在电子温度大于4000K情况下，在同时考虑电子—晶格碰撞和电子—电子碰撞的基础上，构建了改进的TTM。（4）当电子温度超过10000K时，靶材的电子态密度、能带结构发生了变化，更进一步考虑了DOS效应的影响，建立了新的TTM。更多还原

【Abstract】 Pulsed laser deposition （PLD） has received a great deal of attention nowadaysin the world as a promising and versatile technique, especially for growingthin films and preparing nanoparticle field. The research on the experiment isfar beyond the corresponding theoretical research. With the development ofchirp technology and mode-locking technique, high energy and short pulse arethe two main directions of laser development to the extreme conditions. Underthis condition, the rapid development trend of PLD calls for appearance ofcorresponding new mechanism.Our dissertation systematically and intensively focuses on the dynamic ofnanosecond and femtosecond PLD technology, specially, we discuss in detailthe dynamic caused by the the vaporization effect, plasma shielding effect, dynamicabsorption effects in nanosecond pulsed laser ablation （PLA） processing,and non-equilibrium conduction, electron-electron collisions, the effect causedby electron density of states （i.e., DOS effect）, et al. We divide the thermophysicalprocessing into two subprocess which before and after the melting target toestablish more reasonable thermal conductivity dynamics model. More fruitfulresults in femtosecond PLA dynamic research are obtained: we establish a unifiedthermal model of thermophysical effects with pulse width from nanosecondto femtosecond, a new TTM equation includes electron-electron collisions andDOS effect which can be suitable for higher energy field. Using the correspondingnew model, take Ni, Au, Fe, Cu and superconducting YBCO as examples tosimulate their thermal conductivity properties, we find our simulation is moreprecise than the previous research. Our results will be valuable to understand widely the physical representation of PLA processing.The dissertation is organized as follows:The first chapter briefly introduces the PLD technique and correspondingdynamic mechanism, especially the ablation characteristics of nanosecond PLDand femtosecond PLD.The second and third chapters summarize our new results of thenanosecond pulsed laser （include ultraviolet and infrared one） ablationof one-component metal target and multi-elemental oxide superconductingYBa₂Cu₃O₇ （YBCO） target. Firstly, based on the superheat and phase transitionanalysis, two different heat flux equations for different ablation stages, namelybefore and after target melting are presented. Simultaneously considering influenceof vaporization effect, plasma shielding effect, dynamic absorption effects,the results obtained from the two heat flux equations are more reasonable.It must be mentioned that the mean ionization energy we introduced into theequation solve effectively the key part for the plasma shielding effect in multielementtarget. The dynamic development of space- and time-dependence oftemperature in the target is studied. Finally, the influence of vaporization effect,plasma shielding effect, dynamic absorption effects on whole ablation depth arestudied.In the fourth chapter, based on the study of equilibrium ablation fornanosecond laser ablation and non-equilibrium ablation for femtosecond laserablation, the ratio of the electron-phonon coupling timeτ_R and laser pulse widthτ_L is introduced as a smoothly transition parameter for making unified nonfourierthermal model. The space- and time-dependence of electron and latticetemperature of target, and the evolvement of vaporization threshold fluence withlaser pulse width are discussed. In the fifth chapter, the effect of pulse width and fluence of femtosecondlaser on the electron-phonon relaxation time is studied depending on twotemperaturemodel （TTM）. For a certain laser fluence the shorter the pulsewidth, the shorter the electron-phonon relaxation time is. However, the electronphononrelaxation time becomes long for low laser fluence when the pulse widthis fixed.In the sixth and seventh chapters, the great improvement of femtosecondlaser ablation mechanism is studied following the increase of the laser energy.In the sixth chapter, when the electron temperature is higher than 4000K, bothelectron-phonon collisions and electron-electron collisions must be consideredfor describing the improved TTM. Utilizing the improved TTM equations, westudied the dynamic of the electrons and lattices on the target surface followingthe time and the three-dimensional development of the electrons and latticesinside the target following the time and the ablation depth. Moreover, in theseventh chapter, for electron temperature is higher than 10000K, the temperaturedependencies of thermophysical properties are studied for transition metalNi and noble metal Au based on electron density of states （DOS）. The results ofthe analysis of the thermophysical properties at high electron temperatures areincorporated into TTM model and applied for simulations of laser melting ofthin films. The new calculated values of the threshold fluences for surface meltingare in a better agreement with the results of experimental measurements, sothe DOS effect can not be neglected when the electron temperature is higherthan 10000K.The main innovations of the dissertation are as follows:（1） Based on the consideration of vaporization effect, plasma shieldingeffect, dynamic absorption effect, two different heat flux equations for different ablation stages, namely before and after target melting, are presented.（2） A non-fourier unified thermal model is made for the first time, whichcan describe the thermophysical effects with laser pulse width ranges fromnanosecond to femtosecond. The space- and time-dependence of electron andlattice temperature of target, and the evolvement of vaporization threshold fluencewith laser pulse width are discussed in detail.（3） The improved TTM for high energy femtosecond ablation on the conditionthat temperature of electron is higher than 4000K is made by consideringelectron-phonon collisions and electron-electron collisions.（4） A new TTM is established by considering DOS effect under thecondition that temperature of electron is higher than 10000K.更多还原