A theoretical description of high order harmonic generation has been developed that draws upon QED scattering theory and relationships developed previously for multiphoton processes. The framework presented here offers a new perspective on the harmonic generation process and reproduces the salient characteristics of experimental spectra. A direct connection is drawn between harmonic generation and above threshold ionization, and calculated spectra suggest that transitions between (stationary) quantized field Volkov states represent the dominant contribution to the spectrum.

• Received 16 December 1997

DOI:https://doi.org/10.1103/PhysRevLett.81.1833