  We propose to combine the Floquet formalism for systems in ac fields with the
dynamical mean-field theory to study correlated electron systems periodically
driven out of equilibrium by external fields such as intense laser light. This
approach has a virtue that we can nonperturbatively include both the
correlation effects and nonlinear effects due to the driving field, which is
imperative in analyzing recent experiments for photoinduced phase transitions.
In solving the problem, we exploit a general theorem that the Hamiltonian in a
Floquet matrix form can be exactly diagonalized for single-band noninteracting
systems. As a demonstration, we have applied the method to the Falicov-Kimball
model in intense ac fields to calculate the spectral function. The result shows
that photoinduced midgap states emerge from strong ac fields, triggering an
insulator-metal transition.
