THz generation by a two-color pulse in prealigned molecules

We demonstrate that the THz generation in air from a two-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 two-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 in air. For the two-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 two-color pulse with different phase velocities for the orthogonally polarized field components. The plasma effect on the THz generation of the two-color pulse is also observed, leading to additional amplitude and polarization control of the THz radiation.