Formal Scattering Approach to High-Order Harmonic Generation

By using the time-independent formal scattering theoretical approach, we develop a nonperturbative quantum electrodynamics theory to describe high-order harmonic generation (HHG). This theory recovers the semi-classical interpretation of Corkum (Phys. Rev. Lett. 71 (1993) 1994) and gives the same phenomenological cutoff law. The HHG emission rate is expressed as an analytic closed form. We also discuss the connection between HHG and the above threshold ionization from the scattering viewpoint.