  We demonstrate that the THz generation in air from a dual-color pulse,
composed of the fundamental and second-harmonic waves, can be coherently
controlled by field-free molecular alignment. By tuning its time delay to
properly match various molecular alignment revivals, the THz generation from
the dual-color pulse can be promoted or decreased due to the spatial
cross-(de)focusing effect and the alignment-dependent ionization probabilities
of the prealigned diatomic molecules of air. For the dual-color pulse of
orthogonally polarized fundamental and second-harmonic waves, the polarization
of the generated THz radiation can be controlled by the field-free molecular
alignment, which functions as a transient dynamic wave-plate for the dual-color
pulse with different phase velocities for the orthogonally polarized field
components. The plasma effect on the THz generation of the dual-color pulse is
also observed, leading to additional intensity and polarization control of the
THz radiation.
