Highly efficient multi-cycle terahertz radiation driven by counter-propagating modulated wakefields in a homogeneous plasma

We present a novel method for generating narrowband (a few percent) terahertz radiation in plasma, which increases the efficiency of the scheme based on head-on collision of laser-produced plasma wakes by more than an order of magnitude. Instead of exciting narrow wakefields with a width comparable to the plasma skin-depth, we propose to excite wide profiled wakes whose amplitudes are modulated at the same scale by a pair of interfering laser pulses traveling at a small angle to each other. This makes it possible to increase the laser-to-terahertz conversion efficiency up to 1% and to adapt the scheme to the petawatt-class laser systems.