  High-harmonic generation (HHG) of a two-level-system driven by an intense
monochromatic phase-locked laser is studied in terms of complex spectral
analysis with the Floquet method. In contrast to the phenomenological
approaches, this analysis deals with the whole process as a coherent quantum
process based on microscopic dynamics. The spectral decomposition corresponding
to the contributions of the Floquet resonance and dressed continuous states of
the total system have been obtained. The calculated HHG spectrum exhibits the
characteristic features of the HHG from solids. We found that the quantum
interference of the Floquet resonance states is responsible for the transition
from the adiabatic to the stationary regime in the HHG process and that the
phase of the driving laser controls the dynamics of the HHG photon emission.
