We investigate a two-level atom in the field of a strong laser pulse. The resulting time-dependent polarization is the source of a radiation the frequency components of which are essentially harmonics of the driving field's carrier frequency. The time evolution of this secondary radiation is analyzed in terms of the expectation values of the photon-number operators for a large number of electromagnetic modes that are initially in the vacuum state. Our method is based on a multimode version of the Jaynes-Cummings-Paul model and can be generalized to different radiating systems as well. We show that, after the exciting pulse, the final distribution of the photon numbers is close to the conventional (Fourier-transform-based) power spectrum of the secondary radiation. The details of the high-order-harmonic spectra (HHG spectra) are also analyzed; for many-cycle excitations a clear physical interpretation is given in terms of the Floquet quasienergies. A first step towards the determination of the photon statistics of the high-order-harmonic modes reveals states with slightly super-Poissonian distribution.
Gombkötő et al. (Fri,) studied this question.