Quantum electrodynamics treatment of optical high order harmonic generation

Optical high order harmonic generation (HHG) is one of the most promising ways to generate the coherent EUV and soft X-ray radiation required by next the generation of photolithography. It is of equal significance in basic research for understanding the laser-matter interaction. The most intriguing characteristic of HHG spectra is the display of a plateau region, in which the harmonic intensity is relatively insensitive to the harmonic order q, following rapid intensity fall for the lowest harmonics. Several theoretical methods have been explored to describe the nonperturbative behavior of harmonic generation in intense optical fields, which have the unifying theme of calculating dipole radiation from an electron under the influence of the optical field. An alternative framework based on the QED formalism for describing HHG was recently introduced in which the atom and the quantized high optical field was treated as an isolated system with well-defined energy and momentum. The electron-laser interaction is described nonperturbatively by quantized field Volkov states which are modified in a perturbative manner to introduce a harmonic field. As a result, radiation is produced by transitions between quantized Volkov states that experience a phase shift during the process. Such a dressed continuum-to-continuum transition is different from dipole transitions and highlights a direct photon-conversion process in which q fundamental photons are annihilated to produce single harmonic photons with the electron energy unchanged.