  The mechanism of the terahertz (THz) wave generation (TWG) in dual-color
fields is elucidated within the theoretical framework of single-atom based
strong field approximation (SFA). Evaluating the transition dipole moment, the
continuum-continuum (CC) transition, rather than the continuum-bound
recombination for the high-order harmonic generation, is confirmed to be the
core mechanism of the TWG. The analytic form of the SFA-based CC description is
consistent with the classical photoelectric current model, establishing the
quantum-classical correspondence for the TWG. The theory is supported by
parametric dependence of experimental THz yields calibrated by the joint
measurement of the third-order harmonics. Present studies leave open the
possibility of probing the ultrafast dynamics of continuum electron.
