Interaction of an electron beam with a femtosecond laser pulseco-propagating through a wiggler modulates the electronenergies within a short slice of the electron bunch comparable with the duration of the laser pulse (Figure 1). Propagating around an electron storage ring, this bunch develops a longitudinal density perturbation due to the dispersion of electron trajectories. Figure 1 shows how this creates femtosecond electron bunch wings which are used for femtosecond x-ray light. In addition, this density perturbation emits temporally and spatially coherent tera-hertz pulses whichare inherently synchronized to the modulating laser. This gives us a new way to study coherent synchrotron radaition, and creates an opportunity for tuning the THz emmission specifically for the needs of a given experiment
