NER: Nanoscale Photocathodes for Ultrafast Electron Microscopy

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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.