  We develop and exactly solve a model for electrons driven by pulsed or
continuous ac fields. The theory includes both the photoexcitation process as
well as the subsequent acceleration of the electrons. In the case of an ac
response, we examine both the nonequilibrium density of states and the current.
In the case of pulsed light for high harmonic generation, we find the radiated
light assumes a nearly universal behavior, with only limited dependence on the
parameters of the system, except for the amplitude of the driving field, which
determines the range of high harmonics generated and a tendency toward a
narrowing of the peaks in a charge density wave versus a metal. This type of
high harmonic generation can potentially be used for the creation of
solid-state-based ultrafast light sources.
