  We study transport properties of graphene-based p-n junctions irradiated by
an electromagnetic field (EF). The resonant interaction of propagating
quasiparticles with an external monochromatic radiation opens dynamical gaps in
their spectrum, resulting in a strong modification of current-voltage
characteristics of the junctions. The values of the gaps are proportional to
the amplitude of EF. We find that the transmission of the quasiparticles in the
junctions is determined by the tunneling through the gaps, and can be fully
suppressed when applying a sufficiently large radiation power. However, EF can
not only suppress the current but also generate it. We demonstrate that if the
height of the potential barrier exceeds a half of the photon energy, the
directed current (photocurrent) flows through the junction without any dc bias
voltage applied. Such a photocurrent arises as a result of inelastic
quasiparticle tunneling assisted by one- or two-photon absorption. We calculate
current-voltage characteristics of diverse graphene based junctions and
estimate their parameters necessary for the experimental observation of the
photocurrent and transmission suppression.
