Figure 4. Shortest electron pulses.

(a) The laser-electron cross-correlation, integrated for energy gains of 65–80 eV, has a width of 35±3 fs (FWHM). This measurement is limited by the duration of the streaking laser field; hence, the electron pulses are shorter. (b) Measured streaking spectrogram of the compressed pulses; the red arrows denote the lower-energy boundary of where the laser-electron cross-correlation in a is evaluated. E_gain, energy gain. (c) Result of semiclassical simulations, fitted for electron pulse duration and third-order laser dispersion. (d) Sections at 20, 25, 30, 35 and 40 eV show a good agreement between measurement (diamonds) and simulation (blue). (e) Cross-correlation width (diamonds) for each E_gain in comparison with the simulation result assuming 18-fs (purple), 28-fs (blue) or 38-fs (red) electron pulses. (f) Streaking characterization is most reliable at highest energy gains38. The 28-fs pulses (colours as in e) have the best agreement to the measured data. All error bars are s.e.'s.