High-order harmonic generation in an electron-positron-ion plasma*

The laser interaction with an electron-positron-ion mixed plasma is studied, from a perspective of the associated high-order harmonic generation. The harmonic spectrum is shown to be significantly changed after an electron-positron pair plasma is produced. A dense positron beam accelerated by the laser at the target front excites strong counterpropagating plasma waves and triggers inverse two-plasmon decay. Moreover, prominent and well-defined signals at harmonics of the plasma frequency in the high-order harmonic spectrum are efficiently produced. Particle-in-cell simulations with OSIRIS show that significant radiation at twice the plasma frequency can be observed for a pair density as low as $\sim 10^{-5}$ of the electron plasma density. For higher pair densities, the radiation at this characteristic frequency after the pair production can be above $5$ orders of magnitude higher than the counterpart radiation from the plasma without pair production.