A coherent photofield electron source for fast diffractive and point-projection imaging

Prospects for high-resolution imaging at femtosecond speeds using electron diffractive imaging are reviewed in the context of recent achievements using free-electron X-ray lasers. The conflict between Coulomb interactions and the spatial coherence of electron beams is identified as a limiting factor. Experimental results showing the performance of a milliwatt laser-driven fast photofield GaAs electron emitter are presented, including emission current and measured energy spread for various laser energies illuminating the electron emission tip. Band-bending below the Fermi level, due to penetration of the tip field into the emitter, is found to limit the emission energy spread by thermalizing the electrons. Because of the absence of beam crossovers and consequent Coulomb interactions, the point-projection photofield emission microscope with its high spatial coherence is suggested as a method for obtaining femtosecond images at high resolution from atomic processes, which may be triggered repetitively. The incorporation of a photofield emitter into a microwave pulse compression gun is discussed, as is the use of electron photofield emission from semiconductor donor states.