Generation of high-field narrowband terahertz radiation by counterpropagating plasma wakes

It is found that nonlinear interaction of plasma wakefields driven by counterpropagating laser or particle beams can efficiently generate high-power electromagnetic radiation at the second harmonic of the plasma frequency. Using a simple analytical theory and particle-in-cell simulations, we show that this phenomenon can be attractive for producing high-field ($\sim 10$ MV/cm) narrowband terahertz pulses with the gigawatt power level and millijoule energy content.