Femtosecond Electron Sources and Attosecond Electron Foci

We report progress on a nanometer-sized femtosecond electron source. The source consists of an 80-MHz repetition rate femtosecond laser with a tungsten field emission tip. Our autocorrelation spectra support the claim$^{1}$ that the emission process is dependent on the electric field of the laser pulse. This field dependence suggests sub-cycle ($T_{\mbox{cycle}} =2\pi /\omega =2.7\mbox{ fs})$ electron emission.$^{1,2}$ Consistently, we found that the duration of the emission process has an upper bound of 100 fs.$^{3}$ However, we now deduce a lower bound for the emission process duration of about 10 fs. Nevertheless, this electron source may be combined with a temporal lens to focus the electron pulses to subcycle temporal widths.$^{4 }$ $^{1}$P. Hommelhoff, \textit{et} \textit{al}, Phys. Rev. Lett. 96, 077401 (2006) $^{2}$P. Hommelhoff, \textit{et} \textit{al}, Phys. Rev. Lett. 97, 247402 (2006) $^{3}$B. Barwick, \textit{et} \textit{al}, New J. Phys. 9, 142 (2007) $^{4}$P. Baum, A. H. Zewail, Proc. Natl. Acad. Sci. USA. 104, 18409 (2007) * This material is based upon the work supported by the National Science Foundation under Grant Nos. PHY-0355235 and PHY-0653182.