The principle of femtosecond slicing
An intense laser pulse of 30 fs pulse duration and > 1 mJ pulse energy co-propagates with the electron beam through a planar undulator (modulator) which is tuned to emit the laser wavelength (800 nm) in the 1st harmonics, see (a) and (b). As a consequence, some electrons (about 0.1%) in the centre of the bunch expect either gain or loss (~1%) in kinetic energy as they are either accelerated or decelerated by the strong laser field. A subsequent dispersion leads to transverse elongation of electrons which emit synchrotron radiation with larger angular spread in a second undulator (the radiator) few m further downstream. Discriminating this angular part of the beam, only fs x-ray pulses generated by the laser may pass the monochromator. Since the radiator is an elliptical undulator we can generate pulses of variable (even circular) polarization and ~100 fs pulse duration which are naturally synchronized to the laser. By kicking the electrons with a selectable angle through the radiator (c), we can easily select, back and forth, either light from the regular bunch (100 ps) or from the sliced electrons (100 fs).