Transitions between stationary states of the electron-photon field system:A new mechanism for interpreting high optical field phenomena

Optical transitions between stationary states of the electron-photon field system, known as quantum Volkov states, offer a new framework within which to interpret a rich variety of phenomena observed in intense optical fields . In this view, photons and electrons interact to form transient complexes which are capable of annihilating fundamental photons to produce more energ etic radiation, or releasing photons back to the optical field. That is, th ey mediate the exchange of energy between the fundamental and secondary opt ical fields. The application of this new picture, which has no classical an alog, to high harmonic generation is reviewed here. In previous publication s [J. Gao, F. Shen, and J. G. Eden, Phys. Rev. Lett. 81, 1833 (1998); J. Ph ys. B32, 4153 (1999); Phys. Rev. A61, 043812 (2000)], theoretical predictio ns of harmonic spectra and the polarization characteristics of individual h armonics have been compared with semiclassical calculations and experiment and several key results are summarized here. Specifically, an analytic expr ession for the harmonic generation transition matrix element has been deriv ed that is valid for an arbitrary polarization of the fundamental optical f ield and is in agreement with the experimental data in the literature. Also , recent calculations of the spectral lineshapes of individual harmonics ar e presented here and compared with published measurements. QED theory predi cts that the harmonic linewidth is essentially independent of the fundament al intensity when the ponderomotive potential is less than the atomic ioniz ation potential but increases rapidly at higher intensities. The implicatio ns of these results for future experiments are also discussed.