  Linear-response quantum excitation is proportional to the amplitude of the
field, with the energy of the excitation given by the driving frequency. As the
amplitude is increased, there is a crossover, where the excitation energy is
governed by the amplitude of the driving field, not its frequency. As the
amplitude is increased even further, then complex quantum oscillations develop,
whose origin is related to Wannier-Stark physics, but has no simple
explanation. We illustrate this phenomena with the exact solution of the
simplest model of a charge-density-wave insulator driven by a uniform
time-dependent electric field.
