  We study the influence of the polarization states of femtosecond two-color
pulses ionizing gases on the emitted terahertz radiation. A local-current model
and plane-wave evaluations justify the previously-reported impact on the THz
energy yield and an (almost) linearly-polarized THz field when using
circularly-polarized laser harmonics. For such pump pulses, the THz yield is
independent on the relative phase between the two colors. When the pump pulses
have same helicity, the increase in the THz yield is associated to longer
ionization sequences and higher electron transverse momenta acquired in the
driving field. Reversely, for two color pulses with opposite helicity, the
dramatic loss of THz power comes from destructive interferences driven by the
highly symmetric response of the photocurrents lined up on the third harmonic
of the fundamental pulse. While our experiments confirm an increased THz yield
for circularly polarized pumps of same helicity, surprisingly, the emitted THz
radiation is not linearly-polarized. This effect is explained by means of
comprehensive 3D numerical simulations highlighting the role of the spatial
alignment and non-collinear propagation of the two colors.
