  Response of electronic systems in intense lights (AC electric fields) to DC
source-drain fields is formulated with the Floquet method. We have then applied
the formalism to graphene, for which we show that a non-linear effect of a
circularly polarized light can open a gap in the Dirac cone, which leads to a
photo-induced dc Hall current. This is numerically confirmed for a graphene
ribbon attached to electrodes with the Keldysh Green's function.
