时间分辨透射电子显微镜:在纳米区域内观测超快过程

Time-Resolved Transmission Electron Microscopy:Probing Ultrafast Processes on Nanometer Length Scales

超快透射电子显微镜(Ultrafast Transmission Electron Microscopy,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。在哥廷根大学最新的研究进展里,建造了第一台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,获得高度相干的电子脉冲,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。介绍了利用这种先进电子光源UTEM装置的几个应用:对坡莫合金薄膜的磁涡旋纳米图案进行实空间洛伦兹成像,打开应用UTEM进行超快磁性研究的大门;通过将电子脉冲聚焦到数个纳米,我们局域地探测单晶石墨薄膜上飞秒激光激发的声学声子在边缘的传播和演化;演示了自由传播电子束在激光驱动的近场中受光学相位调制产生的电子动量态相干叠加。

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 Gttingen UTEM instrument.We demonstrate real-space Lorentz imaging of magnetic vortex structures in a nanopatternedpermalloy film,opening up the investigation of ultrafast magnetism in UTEM.With electron pulses focused to a few nanometers,we locally probe laser-induced acoustic phonons,originating from the edge of a single-crystalline graphite flake.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.