Saturation of Terahertz Generation by Multicolor Ionizing Pulses

The optimal waveform of an ionizing pulse and the corresponding maximum values of the density of residual terahertz currents are found analytically. The solutions obtained can be realized using multicolor femtosecond pulses containing a field at the fundamental frequency and several of its initial harmonics. The amplitudes of odd harmonics in the optimal field are larger than those of neighboring even harmonics. The maximum residual current density increases with the number of harmonics, the fundamental wavelength, and the ionization potential of gas particles and decreases with the duration of the ionizing pulse, approaching the ceiling values when few-cycle pulses are used.