Electron beam driven generation and amplification of isolated relativistic sub-cycle pulses*

We propose a scheme for the generation of very intense sub-cycle pulses. Intense sub-cycle pulses find applications from reaction dynamics at the electronic level to the generation of isolated attosecond and zeptosecond X-ray pulses. However, methods like the parametric amplification are limited in terms of output energy to some tens of uJ due to material damage thresholds. To overcome these limitations we propose a method in which a pump electron beam is injected into an electromagnetic seed pulse as the latter is reflected by a mirror. The electron beam is shown to amplify the field of the seed pulse while upshifting its central frequency and reducing its number of cycles. We demonstrate the amplification by means of 1D and 2D particle-in-cell simulations. In order to explain and optimize the amplification process, a model based on fluid theory is proposed. We estimate that using currently available electron beams and terahertz pulse sources, our scheme is able to produce mJ-strong mid-infrared sub-cycle pulses.