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. 
I. V. Timofeev, V. V. Annenkov, E. P. Volchok
physics.plasm-ph