Formal scattering approach to high-order harmonic generation

By using the time-independent formal scattering theoretical approach, we de velop a nonperturbative quantum electrodynamics theory to describe high-ord er harmonic generation (HHG). This theory recovers the semi-classical inter pretation of Corkum (Phys. Rev. Lett. 71 (1993) 1994) and gives the same ph enomenological cutoff law. The HHG emission rate is expressed as an analyti c closed form. We also discuss the connection between HHG and the above thr eshold ionization from the scattering viewpoint.