  We propose a novel scheme for frequency-tunable sub-cycle electromagnetic
pulse generation. To this end 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 be able 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 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.
