Generation of terahertz radiation by multicolor ionizing pulses

Analytical formulas have been obtained for the optimal waveform of an ionizing pulse and corresponding maximum density of residual terahertz currents. The found analytical solutions can be implemented with multicolor femtosecond pulses containing the field at the fundamental frequency and several of its lowest harmonics. The amplitudes of odd harmonics in the field optimal for the generation of terahertz radiation are larger than those for neighboring even harmonics. The maximum residual current density increases with an increase in the number of harmonics, in the wavelength of the fundamental harmonic, and in the ionization potential of gas particles and with a decrease in the duration of the ionizing pulse. This maximum residual current density approaches limiting values for saturated ionization when few-cycle pulses are used.