  High-field terahertz (THz) pulse generation is investigated through the
interaction of an intense single-color CO2 laser pulse with helium (He) gas
targets. Employing multi-dimensional Particle-In-Cell (PIC) simulations, this
study reveals a substantial enhancement in THz generation efficiency, even with
a single-color laser pulse interacting with gas targets in the
self-modulated-laser-wakefield (SMLWF) regime. Our study demonstrates that in
the presence of photoionization, a synergistic interplay of laser
self-modulation, self-focusing, and local pump depletion leads to the
generation of robust THz pulses polarized parallel to the laser electric field.
The dependence of THz generation efficiency on target density and laser pulse
duration has been investigated. Our study identifies a favourable parametric
regime for producing THz fields with amplitudes reaching hundreds of GV/m,
surpassing those reported in previous studies.
