  The terahertz (THz) generation in a dual-color field was investigated
experimentally by precisely controlling the relative time delay $\tau$ and
polarization $\theta$ of dual-color lasers, where the accompanying
third-harmonic generation (THG) is employed to determine $\tau$ up to the
sub-wavelength accuracy. An anticorrelation of the yields between THz and THG
with $\tau$ was displayed. Compared with the theoretical simulations, the
experimental results reveal that the continuum-continuum transitions of the
released electron after single-atom ionization are the dominating mechanism for
THz generation, which is opposite to the high-harmonic generation (HHG) based
on the recollision mechanism, or the radiation induced by perturbative
bound-bound transitions.
