NER: Nanoscale Photocathodes for Ultrafast Electron Microscopy

2004年4月16日至17日，在加州普莱森顿举行的第一届国家实验室和大学联盟超快电子显微镜研讨会明确指出，将电子显微镜的原子尺度分辨率与现在超快激光光谱学标准的亚皮秒（10-12秒）时间分辨率结合起来，将促进巨大的潜在科学进步。动态透射电子显微镜（DTEM）的发展提供了前所未有的时空分辨率，为纳米尺度上的材料和分子传感提供了重要的新工具。在未来，这样一个创新的进步可以让单个原子运动的直接可视化——这是理解化学反应动力学（例如，生物和工业催化）和固态材料的基本物理过程（例如，晶格振动与原子尺度缺陷的相互作用）的重要一步。即使这种仪器提供的关于单个纳米粒子和结构的有限的时间信息，也应该能够在纳米级器件和系统架构方面取得重大改进。正是这种具有重大科学效益和广泛影响的前景，改善了这一具有技术挑战性的探索性研究项目的高风险。我们打算探讨未来科技发展面临的首要关键智力问题；即，在存在本征退相干电子-电子散射（或空间电荷）效应的情况下，从超短脉冲激光驱动的光电阴极产生空间相干电子脉冲。为了应对这一挑战，我们建议将当今的纳米技术与最先进的高功率皮秒激光系统相结合，以控制大面积纳米图案（例如量子点阵列）光电阴极的脉冲光电发射的初始空间相位。研究了原型超快电子枪电子脉冲的传播动力学（即部分空间相干性和时间脉冲展宽），以确定最佳相干光发射条件。

The recent First National Lab and University Alliance Workshop on Ultrafast ElectronMicroscopies, April 16-17, 2004, Pleasanton, CA, clearly indicated the tremendous potential scientific advances that would be facilitated by combining today's atomic-scale resolution of electron microscopy with the sub-picosecond (10-12s) temporal resolution now standard in ultrafast laser spectroscopy. The unprecedented spatio-temporal resolution afforded by the development of a dynamic transmission electron microscope (DTEM) would provide an important new tool for sensing materials and molecules on the nanoscale. Such an innovative advance could, in the future, allow the direct visualization of individual atomic motions - an important step in understanding both chemical reaction kinetics (e.g., biological and industrial catalysis) and fundamental physical processes in solid-state materials (e.g., the interaction of lattice vibrations with atomic-scale defects). Even modest temporal information provided by such an instrument on individual nano-particles and structures should also enable significant improvements to be made in Nanoscale Devices and System Architecture. It is this promise of significant scientific benefit and broad impact that ameliorates the high risk of this technologically challenging exploratory research project.We intend to investigate the primary key intellectual problem facing the development of a future DTEM; namely, the generation of a spatially-coherent electron pulse from an ultrashort pulse laser-driven photocathode in the presence of intrinsic decoherencing electron-electron scattering (or space-charge) effects. To meet this challenge, we propose to combine today's nanotechnology with a state-of-the-art, high-power, picosecond laser system to control the initial spatial phase of pulsed photoemission from a large-area nano-patterned (e.g., quantum dot array) photocathode. The propagation dynamics (i.e., partial spatial coherence and temporal pulse broadening) of the electron pulse from the prototype ultrafast test-bed electron gun will be studied to determine the optimum coherent photoemission conditions.