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Femtosecond electron pulse as an ultrafast probe.

机译：飞秒电子脉冲作为超快探针。

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摘要

The thesis presents the recent development of the 3rd generation femtosecond electron diffractometer in Professor Jim Cao's group and its application to study ultrafast processes in real time. The research activities cover two main subjects: photoinduced structural phase transition (PIPT) in colossal magnetoresistive (CMR) materials and the dynamics of electron emission and the associated residual charge redistribution in targets during the early stage of laser ablation. In the study of PIPT in CMR materials, a direct and real time measurement of photoinduced structure phase transition in single crystal La0.84Sr0.16MnO3 and LaMnO3 was performed by using femtosecond electron diffraction. The melting of orthorhombic lattice ordering under femtosecond optical excitation is found involving two distinct processes with different time scales, an initial fast melting of orthorhombic phase in 3 ps and a subsequent much slower transformation in 50 ps and longer timescales. The fast process can be attributed to the initial melting of orthorhombic phase induced by the Mn-O bond change that is driven by the quenching of the Jahn-Teller distortion following the photo-excitation. The slow process is associated with the growing of newly formed structure domain from excited sites to the neighboring non-excited orthorhombic sites.;In the second project, two new techniques, namely femtosecond electron shadow imaging and ultrafast electron deflectometry, were developed. These two complementary techniques provide both a global view and local prospect of the associated transient electric field and charge expansion dynamics. The results reveal that the charge cloud above the target surface is predominantly composed of thermally ejected electrons and the charge cloud expands with a fast front-layer speed exceeding 107 m/s. The average electric field strength of the charge cloud induced by a pump fluence of 2.2 J/cm 2 is estimated to be on the order of ∼2.4x105 V/m. For the temporal evolution of residual charges on the target, the results show that residual charges in metals can redistribute themselves almost instantly, abiding by the boundary conditions and Maxwell equations in the same way as they would at electrostatic equilibrium condition. However, residual charges in dielectrics are confined within the excited area for hundreds of picoseconds and beyond. These observations provide an experimental support to the alleged coulomb explosion phenomenon in previous studies, as well as a reference for modeling residual charge dynamics.;In addition, a 1-D molecular dynamics simulation of coherent lattice motion in laser excited thin film is presented in the last section of this thesis. Using this simulation, both the displacement and expansion at each lattice site along the 1-D atomic chain can be traced as a function of delay time. In particular, the simulation shows that the electronic thermal stress is responsible for driving the lattice motion at the early stage, which matches very well with our FED experimental data obtained in the study of ultrafast heating of free-standing metal films.
机译：本文介绍了第三代飞秒电子衍射仪在曹Jim教授团队中的最新进展及其在实时研究超快过程中的应用。研究活动涵盖两个主要主题：巨大磁阻（CMR）材料中的光致结构相变（PIPT）以及激光烧蚀早期目标中电子发射的动力学以及相关的残余电荷再分布。在CMR材料中PIPT的研究中，通过飞秒电子衍射对La0.84Sr0.16MnO3和LaMnO3单晶中的光致结构相变进行了直接和实时的测量。飞秒光学激发下的正交晶格有序熔化被发现涉及两个不同的过程，它们具有不同的时间尺度，正交相的初始快速熔化为3 ps，随后的慢得多的转变为50 ps和更长的时间尺度。快速过程可以归因于由Mn-O键变化引起的正交晶相的初始熔化，这是由光激发后Jahn-Teller畸变的淬灭驱动的。缓慢的过程与新形成的结构域从激发位点到邻近的非激发正交晶位点的生长有关。在第二个项目中，开发了两种新技术，即飞秒电子阴影成像和超快电子偏转法。这两种互补的技术提供了相关瞬态电场和电荷扩展动力学的全局视图和局部前景。结果表明，目标表面上方的电荷云主要由热喷射电子组成，并且电荷云以超过107 m / s的快速前层速度扩展。由2.2 J / cm 2的泵浦注量引起的电荷云的平均电场强度估计约为2.4x105 V / m。对于目标上残余电荷的时间演化，结果表明，金属中的残余电荷几乎可以立即重新分布，遵守边界条件和麦克斯韦方程组的方式与在静电平衡条件下相同。然而，电介质中的残留电荷被限制在受激区域内数百皮秒甚至更长时间。这些观察结果为先前研究中所谓的库仑爆炸现象提供了实验支持，并为建模残余电荷动力学提供了参考。此外，本文还对激光激发薄膜中相干晶格运动的一维分子动力学模拟进行了研究。本文的最后一部分。使用该模拟，可以追踪一维原子链上每个晶格位点的位移和扩展随延迟时间的变化。特别是，仿真表明，电子热应力是驱动晶格运动早期的原因，这与我们在自立式金属膜的超快加热研究中获得的FED实验数据非常吻合。

著录项

  • 作者

    Li, Junjie.;

  • 作者单位

    The Florida State University.;

  • 授予单位 The Florida State University.;
  • 学科 Physics General.;Physics Optics.
  • 学位 Ph.D.
  • 年度 2011
  • 页码 108 p.
  • 总页数 108
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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