  We investigate theoretically and numerically the light-matter interaction in
a two-level system (TLS) as a model system for excitation in a solid-state band
structure. We identify five clearly distinct excitation regimes, categorized
with well-known adiabaticity parameters: (1) the perturbative multiphoton
absorption regime for small driving field strengths, and four light
field-driven regimes, where intraband motion connects different TLS: (2) the
impulsive Landau-Zener (LZ) regime, (3) the non-impulsive LZ regime, (4) the
adiabatic regime and (5) the adiabatic-impulsive regime for large electric
field strengths. This categorization is tremendously helpful to understand the
highly complex excitation dynamics in any TLS, in particular when the driving
field strength varies, and naturally connects Rabi physics with Landau-Zener
physics. In addition, we find an insightful analytical expression for the
photon orders connecting the perturbative multiphoton regime with the light
field-driven regimes. Moreover, in the adiabatic-impulsive regime, adiabatic
motion and impulsive LZ transitions are equally important, leading to an
inversion symmetry breaking of the TLS when applying few-cycle laser pulses.
This categorization allows a deep understanding of driven TLS in a large
variety of settings ranging from cold atoms and molecules to solids and qubits,
and will help to find optimal driving parameters for a given purpose.
