Quantum mechanical approach to probing the birth of attosecond pulses   using a two-color field
  We investigate the generation of even and odd harmonics using an intense laser and a weak second harmonic field. Our theoretical approach is based on solving the saddle-point equations within the Strong Field Approximation. The phase of the even harmonic oscillation as a function of the delay between fundamental and second harmonic field is calculated and its variation with energy is found to be in good agreement with recent experimental results. We also find that the relationship between this phase variation and the group delay of the attosecond pulses, depends on the intensity and wavelength of the fundamental field as well as the ionization potential of the atom. 
J. M. Dahlstr\"om, A. L'Huillier and J. Mauritsson
quant-ph physics.atom-ph physics.optics