【作者】 许孝芳；

【导师】 周明；

【作者基本信息】 江苏大学 ， 材料学， 2013， 博士

【摘要】 随着激光技术的发展,飞秒激光技术在能源、材料、信息技术和微加工等领域中有着广泛的应用,其中飞秒激光作用下材料内能量转换与传递过程的微观机理是重要的研究内容。能量转换与传递过程在微观层面上体现为载流子间的相互作用,其作用时间极短,一般都处在皮秒至飞秒量级。飞秒激光瞬态热反射(FTTR)技术通常又称为飞秒激光泵浦-探测技术,可以用来分辨并观测这些载流子间的相互作用过程,是观测微观能量转换与传递过程及对其规律进行研究的主要实验手段。本文主要研究了飞秒激光对金属薄膜进行加热后,金属薄膜表面的瞬态反射率的变化现象,详细讨论了金属内电子的热输运过程。主要概括如下：1.简单介绍了双温模型、三温模型和非热模型三种针对超短脉冲激光与物质相互作用的模型,用以描述金属内不同载流子之间的非平衡态热输运过程。2.介绍了进行泵浦-探测实验的准备工作,包括实验系统的搭建和实验样品的制备两部分。讨论了实验系统中的主要仪器的选择,介绍了主要仪器的参数及其工作原理；介绍了磁控溅射镀膜原理、镀膜设备以及退火原理和退火设备等。3.对Ag薄膜、Cr薄膜、Co薄膜和NiFe合金薄膜进行了单泵浦光束的泵浦-探测实验,阐述了具体实验过程,描述了薄膜的瞬态反射率曲线的变化趋势,分析了薄膜内电子和声子的运动规律,重点讨论了当电-声子发生非平衡弛豫时,通过探测薄膜表面的反射光而得到的瞬态反射率曲线的变化规律以及电子的热化时间、加热时间等的变化。另外,在以玻璃作为衬底的Cr薄膜和NiFe合金薄膜上发现了多次瞬态反射率突变现象,这个现象在使用单束泵浦激光光束加热的泵浦-探测实验中很少被详细的讨论。4.为深入研究多次瞬态反射率突变现象,搭建了双泵浦光束的实验系统。对Pt薄膜和Ag薄膜进行了双光束泵浦-探测实验研究。在对Pt薄膜上的瞬态反射率信号的研究中,发现在20nm的玻璃衬底的Pt薄膜的瞬态反射率曲线上出现了一个在单泵浦光束加热的泵浦-探测实验中很少出现过的现象：薄膜在双飞秒激光泵浦光束的作用下,ΔR/R曲线急剧下降到一个低值后,在最长达到约130ps的延迟时间内ΔR/R曲线维持在低反射率状态,随后ΔR/R曲线再上升到热平衡状态。由于薄膜内的电子温度与薄膜的瞬态反射率值成正比,所以也表明Pt薄膜内的电子温度在130ps的时间内维持在低温状态。另外在对Ag薄膜进行双泵浦光束实验时,在小于15ps的时间延迟内发现了多至4次的瞬态反射率突变现象,说明在小于15ps的延迟时间内Ag薄膜内的电子温度发生了多次急剧的上升或下降。这些现象的出现对研究飞秒激光与物质相互作用的机理有着重要的意义。更多还原

【Abstract】 With the development of laser technology, femtosecond laser is widely used in energy, materials, information technology and micro machining field. The microscopic mechanism of energy conversion and transfer process in materials by femtosecond laser heating is an important research content. The energy conversion and transfer process embody the interactions between the carriers at a micro level, the time scale of the interactions is very short, generally in picosecond to femtosecond level. The femtosecond transient reflection technique (FTTR) is often called the femtosecond pump probe technique, it can distinguish and observe the interactions between the carriers, so it is the main experimental method to observe and research the micro energy transfer processing and the regularities. This paper mainly studied on the transient reflectivity changes on some metal films’surface, and discussed the heat transport process of electron-electron and electron-phonon in the metal films by femtosecond laser heating. It was summarized as follows:1. We introduced three kinds of models when the ultrafast pulses laser heat the metal films:the two temperature model, the three temperature model and the nonthermal model. They aimed at the nonequilibrium heat transport process of different carriers in metal films.2. The preparations of the pump probe experiment was introduced in details. It contained two parts:the building of the pump probe experiment system and the preparation of samples. The working principle of the main instruments was discussed in details, and also the samples preparing using magnetron sputtering method was introduced, which included the principle of plating, the coating equipment and the annealing equipment etc..3. We performed pump probe experiments by using single pump beam to heat Ag films, Cr films, Co films and NiFe alloy films. We expounded the specific experimental process, described the trend of the transient reflectivity curve, and analyzed the movement of electrons and phonons in the thin films. We discussed the variation regularities of transient reflectivity curves for the electron-phonon nonequilibrium relaxation occuring, and analysis the electron thermalization time, the heating time et al.. We found and discussed the multiple transient reflectivity mutation phenomenon on the Cr films and NiFe alloy films of glass substrates, which were not discussed in detail in other pump probe experiments when the pump beam was single.4. In order to deeply analysis the multiple transient reflectivity mutation phenomenon, we set up the experimental system of double pump beams. The Pt films and the Ag films were also studied by pump probe technology. By analysis the transient reflectivity signals on Pt films, we found that the transient reflectivity curves of platinum film of20nm on glass substrates had a different phenomenon:after the Pt films heated by the double laser pulses, the AR/R curves sharply declined to a low value, the lasting time was approximately130ps when the ΔR/R curves remained at the low reflectivity level, and then the ΔR/R curves rose to the thermal equilibrium state. The phenomenon was not experienced in single pump beam experiments. Due to the electron temperature of films was proportional to the transient reflectivity of the films, so the electron temperature of the Pt film maintain at low temperature state in130ps. In addition, when the silver films were heated by the double pump beams, the transient reflectivity changed sharply up to4times in the delay time of less than15ps, so the electron temperature of the Ag thin films rapidly increased or declined many times in less than15ps. In a word, the emergence of these phenomena was important to research the interactions of femtosecond laser and metals.更多还原