  We investigate high-harmonic generation in closed systems, using the
two-level atom as a simplified model. By means of a windowed Fourier transform
of the time-dependent dipole acceleration, we extract the main contributions to
this process within a cycle of the driving field. We show that the patterns
obtained can be understood by establishing a parallel between the two-level
atom and the three-step model. In both models, high-harmonic generation is a
consequence of a three-step process, which involves either the continuum and
the ground state, or the adiabatic states of the two-level Hamiltonian. The
knowledge of this physical mechanism allows us to manipulate the adiabatic
states, and consequently the harmonic spectra, by means of a bichromatic
driving field. Furthermore, using scaling laws, we establish sharp criteria for
the invariance of the physical quantities involved. Consequently, our results
can be extended to a broader parameter range, as for instance those
characteristic of solid-state systems in strong fields.
