  Sources of intense, ultra-short electromagnetic pulses enable applications
such as attosecond pulse generation, control of electron motion in solids and
the observation of reaction dynamics at the electronic level. For such
applications both high-intensity and carrier envelope phase~(CEP) tunability
are beneficial, yet hard to obtain with current methods. In this work we
present a new scheme for generation of isolated CEP-tunable intense sub-cycle
pulses with central frequencies that range from the midinfrared to the
ultraviolet. It utilizes an intense laser pulse which drives a wake in a
plasma, co-propagating with a long-wavelength seed pulse. The moving electron
density spike of the wake amplifies the seed and forms a sub-cycle pulse.
Controlling the CEP of the seed pulse, or the delay between driver and seed
leads to CEP-tunability, while frequency tunability can be achieved by
adjusting the laser and plasma parameters. Our 2D and 3D Particle-In-Cell
simulations predict laser-to-sub-cycle-pulse conversion efficiencies up to 1%,
resulting in relativistically intense sub-cycle pulses.
