A new (“linear-parametric”) mechanism of a direct conversion of an ultrashort laser pulse into terahertz radiation is suggested. The conversion is due to the ionization-induced excitation and the subsequent electromagnetic emission of the superluminous polarization wave created by the axicon-focused laser pulse. For a few-cycle pulse with an optimum carrier-envelope phase, the considered mechanism is found to be much more effective than the alternative one based on the excitation of plasma oscillations in the laser wakefield by the ponderomotive force and able to provide THz radiation of the gigawatt power level with the use of moderate optical intensity ($\sim {10}^{14}–{10}^{15}\mathrm{W}/{\mathrm{cm}}^2$).

• Received 29 March 2007

DOI:https://doi.org/10.1103/PhysRevLett.98.245002