Sequential terahertz pulse generation by photoionization and coherent transition radiation in underdense relativistic plasmas

Terahertz (THz) emission by two-color, ultrashort optical pulses interacting with underdense helium gases at ultrahigh intensities ($> 10^{19}\,\mathrm{W/cm}^2$) is investigated by means of 3D particle-in-cell simulations. The THz field is shown to be produced by two mechanisms occurring sequentially, namely, photoionization-induced radiation (PIR) by the two-color pulse and coherent transition radiation (CTR) by the wakefield-accelerated electrons escaping the plasma. For plasmas of atomic densities $> 10^{17}\,\mathrm{cm}^{-3}$, CTR proves to be the dominant process, providing THz bursts with field strength as high as $100\,\mathrm{GV/m}$ and energy in excess of $1\,\mathrm{mJ}$. Analytical models are developed for both the PIR and CTR processes, which correctly reproduce the simulation data.