Classical harmonic generation in rare gases

The classical microcanonical ensemble approach to high-harmonic generation (HHG) in rare gases subjected to intense laser fields is studied. We show that the ensemble spectrum is a "sampled" version of the single trajectory spectrum. Unlike the radiation of the single ensemble member, the total ensemble radiation possesses all the basic HHG features: odd laser harmonics, plateau, and cutoff. The sampling theorem for uniform grids is used to explain why the ensemble spectrum can be computed accurately with a very small number of ensemble members compared to the Monte Carlo method. Furthermore, The phase space relevant to harmonic generation is found to be significantly smaller than the field free microcanonical ensemble. In addition we demonstrate the seeding effect that was predicted and observed in quantum simulation. For circular polarization, we verify that the harmonic generation is highly suppressed even when the argument of the three-step model does not apply. All the findings are numerically calculated for the xenon atom.