武汉光电论坛第106期

报告题目：时间分辨透射电子显微镜：探测纳米尺度的超快过程
Time-resolved transmission electron microscopy: Probing ultrafast processes on nanometer length scales

时 间：2015年11月13日10:00-12:00
地 点：武汉光电国家实验室A101
报 告 人：Sascha Schäfer 教授，德国哥廷根大学
邀 请 人：梁文锡 教授

报告人简介：
Schäfer博士于2004年在德国Technical University Darmstadt (TUD)化学系本科毕业，获得优等学位；2008年在TUD取得博士学位，获优等学位；2009-2012在加州理工学院(Caltech)A. H. Zewail教授的研究组做博士后；2012年至今在哥廷根大学做Habilitand。Schäfer博士的研究兴趣包括超快电子、磁性和结构动力学过程，超快电子显微镜技术与相关研究，以及纳米尺度的超快激光与物质相互作用等。他在哥廷根大学成功建造了性能优越的超快电子显微镜和超快低能电子衍射装置，相关工作发表在Science和Nature等刊物上。作为青年学者，Schäfer博士获得了一系列的荣誉，包括中国学界熟悉的洪堡奖学金。

Biography:
Dr. Schäfer graduated at the Technical University Darmstadt (TUD) with honors in 2004; he received his Ph. D. diploma with honors at TUD in 2008. In 2009-2012 he worked as a post doctoral scholar in A. H. Zewail’s group at Caltech. Since 2012 he has been working for Habilitand at Georg-August-Universität Göttingen. Dr. Schäfer’s research interests include ultrafast electronic, magnetic and structural dynamics in inhomogeneous systems, methodical development of time-resolved electron microscopy and diffraction, and light-matter interaction at the nanoscale. He built the state-of-art ultrafast transmission electron microscopy and ultrafast low energy electron diffraction in Georg-August-Universität Göttingen. His work were published in high profile journals including Science and Nature. As a young scholar, Dr. Schäfer received many honors, such as the Feodor-Lynen-Fellow of the Alexander von Humboldt-Gesellschaft.

报告摘要：
超快电子显微镜(UTEM)是一种能够以纳米尺度空间分辨研究超快动力学过程的前沿技术。在我们最新的进展里，我们建造了具有高度相干性电子源的第三代UTEM。通过从纳米针尖发射局域的光电子，我们能够在样品处将电子斑聚焦到数个纳米，同时具有300fs的脉冲时间宽度。
在这个报告里我将介绍利用这种先进电子光源的几个应用：1）实空间的洛伦兹成像，打开应用UTEM进行超快磁性研究的大门；2）局域探测石墨薄膜的声学声子；3）自由电子束的光学相位调控在电子的动量空间中产生相干叠加。最后，我们讨论第三代UTEM在未来的广泛应用。

Abstract:
Ultrafast transmission electron microscopy (UTEM) is a promising technique for the investigation of ultrafast dynamics with nanoscale spatial resolution. Based on the recent progress in the construction of coherent pulsed electron sources, we present the first implementation of a 3rd-generation UTEM instrument. Utilizing localized photoemission from a nanoscale needle photocathode, we obtain highly coherent electron pulses with electron focal spot sizes on the sample of a few nanometers and electron pulse durations of about 300 fs.
In this colloquium, we will present several applications enabled by the advantageous beam properties in the Göttingen UTEM instrument: (1) we demonstrate real-space Lorentz imaging of magnetic vortex structures in a nanopatterned permalloy film, opening up the investigation of ultrafast magnetism in UTEM. (2) we locally probe laser-induced acoustic phonons, originating from the edge of a single-crystalline graphite flake. (3) we demonstrate optical phase modulation of free electron beams in laser-driven near-fields, yielding coherent superpositions of electron momentum states. Finally, I will discuss future research directions of 3rd-generation UTEM instruments, using their broad capabilities for time-resolved electron imaging, diffraction and spectroscopy.

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