Terahertz generation in dual-color laser: continuum electron in strong-field single-atom ionization

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.