Relativistic ultrahigh-order harmonic generation

A recently proposed quantum theory of relativistic ultrahigh-order harmonic generation [Milosevic, D.B., Hu, SX, and Becker, W., 2001, Phys. Rev. A, 63, 011403(R)] is analyzed in more detail. Several variants of this theory are presented. Our numerical results show the appearance of a multiplateau structure in the harmonic spectrum in the weakly relativistic regime. For stronger fields, the interference pattern familiar from the non-relativistic case disappears and the height of the plateau decreases towards high harmonic orders. The cutoff of the harmonic spectrum is slightly lower compared to the non-relativistic case. These features are confirmed for stronger fields and explained in terms of complex relativistic quantum orbits.