Harmonic generation from laser-driven vacuum

We investigate the feasibility that, in the field of a superstrong standing laser wave, high-order harmonics of the pumping laser are generated from vacuum. Analytical calculations employing adiabatic perturbation theory show that, for laser electric fields larger than E{sub cr}=m{sup 2}c{sup 3}/({Dirac_h}/2{pi})e=1.3x10{sup 16} V/cm, high-order harmonics are generated. The harmonic spectrum shows a wide plateau followed by a cutoff. The cutoff starts approximately at photon energies ({Dirac_h}/2{pi}){omega}{sub M}{approx}{radical}(({Dirac_h}/2{pi})ecE{sub L}), with E{sub L} being the amplitude of the laser field. In the opposite limit E{sub L}