  In order to investigate photoinduced insulator-metal transitions observed in
correlated electron systems, we propose a new theoretical method, where we
combine a Floquet-matrix method for AC-driven systems with the dynamical
mean-field theory. The method can treat nonequilibrium steady states exactly
beyond the linear-response regime. We have applied the method to the
Falicov-Kimball model coupled to AC electric fields, and numerically obtained
the spectral function, the nonequilibrium distribution function and the
current-voltage characteristic. The results show that intense AC fields indeed
drive Mott-like insulating states into photoinduced metallic states in a
nonlinear way.
