Influence of the Terahertz Field on the Processes of Low- and High-Order Harmonic Generation by Femtosecond Laser Pulses in a Gaseous Medium

The influence of a strong (up to 7.5 MV/cm) terahertz field on the processes of low (2-5 eV) and high (55-83 eV) order harmonic generation of the near-infrared radiation with a wavelength of 1.24 mu m under the excitation of an argon jet has been experimentally investigated. It has been shown that the quasistatic terahertz field leads to the appearance of even low- and high-order harmonics, which is due to the violation of the spatial symmetry of the atomic potential in the first case and to the terahertz-induced breaking of the infrared waveform symmetry in the second case. It has been experimentally observed that the terahertz field leads to the simultaneous modulation of the intensity of both even and odd high-order harmonics, while a change in the intensity of low-order harmonics has been observed only for even components. Even low (2nd) and high (70th) order harmonics demonstrate different power-law dependences on the terahertz field strength under an increase in the infrared pulse energy, which is due to the appearance of additional radiation at the frequency of the second harmonic under the formation of laser-induced plasma.