  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.
