Generation of multi-cycle relativistic terahertz radiation from photon deceleration in a long self-induced plasma wake

Intense terahertz (THz) radiation with multiple cycles is of great interest in many active research fields ranging from particle acceleration to space communication. However, its generation remains a challenge since multi-cycle pulses produced from crystal-based or plasma-based methods are usually characterized by non-relativistic intensity, while plasma-based techniques can deliver much more intense pulses but with few cycles. In this study, we propose a scheme to generate THz radiation with relativistic intensity and multi-cycle duration by shooting a ∼100 TW laser pulse into an underdense plasma with a structured density profile. In this process, laser photons are decelerated by the co-moving refractive index gradient formed by the laser–plasma interaction and slide back to be stored in the self-induced long plasma wake. Particle-in-cell simulations demonstrate the generation of THz radiation with a frequency of 21.4 THz, a pulse length of ∼100 μm (∼7 cycles), a pulse energy up to 18.5 mJ, and an energy conversion efficiency of ∼2%. These pulses may enable access to unexplored regimes of strong THz-material interaction, or open up possibilities for relativistic THz optics.