High-order harmonic generation by multielectron atoms in the field of a relativistic-intensity standing electromagnetic wave

A semiclassical model of high-order harmonic generation (HHG) is developed and applied for the investigation of HHG in relativistic-intensity laser fields. A shift of photo-electrons along the wave vector of the pump wave is shown to reduce the harmonic intensity. In the low-frequency range of the spectrum of high-order harmonics, this intensity lowering is more noticeable than in the high-frequency range. We propose to use a standing wave instead of a travelling wave to increase the HHG efficiency by reducing the longitudinal shift of photoelectrons. The efficiency of using atoms in a standing wave is shown to decrease with the growth in the pump intensity. However, in the case of a travelling wave, this decrease in the HHG efficiency is even more noticeable. The use of a standing wave provides a considerable increase in the HHG efficiency, which may be as high as ~ 10² when pump radiation has an intensity I ≈ 10¹⁸ W cm^-2 and a wavelength of 0.3 μm.