Relativistic ultrahigh-order harmonic generation
A recently proposed quantum theory of relativistic ultrahigh-order harmonic generation [Milosevic, D.B., Hu, S.X., 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 nonrelativistic 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 nonrelativistic case. These features are confirmed for stronger fields and explained in terms of complex relativistic quantum orbits.