High-order harmonic generation (HHG) in laser fields of relativistic intensity is studied. It is shown that, owing to relativistic longitudinal displacement of the photoelectrons, significant HHG suppression occurs. Such suppression is greater for the low-frequency part of the HHG spectrum than for the high-frequency part. We propose to use a standing-wave pump instead of a traveling wave to overcome this effect and to enhance the efficiency of HHG. The efficiency of utilizing atoms in a standing wave decreases with growth in the pump intensity. However, in traveling-wave HHG the decrease in efficiency is still greater; therefore, using a standing wave allows one to obtain a HHG intensity that is essentially greater than in a traveling wave (for example, a gain factor of 10<sup>2</sup> can be achieved when one is pumping Ar<sup>8+</sup> ions with a radiation of intensity 10<sup>18</sup> W/cm<sup>2</sup> and a wavelength of 0.3 μm).
© 2002 Optical Society of America
(190.4160) Nonlinear optics : Multiharmonic generation
(190.4180) Nonlinear optics : Multiphoton processes
(270.4180) Quantum optics : Multiphoton processes
(270.6620) Quantum optics : Strong-field processes
(350.5720) Other areas of optics : Relativity
Vladimir D. Taranukhin and Nickolay Yu. Shubin, "High-order harmonic generation by atoms with traveling- and standing-wave pumps of relativistic intensity," J. Opt. Soc. Am. B 19, 1132-1140 (2002)