Transport in graphene exposed to a strong electromagnetic field

We study quasiparticle dynamics in graphene exposed to a linearly-polarized electromagnetic wave of very large intensity. We demonstrate that low-energy transport in such system can be described by an effective time-independent Hamiltonian, characterized by multiple Dirac points in the first Brillouin zone. Around each Dirac point the spectrum is anisotropic: the velocity along the polarization of the radiation significantly exceeds the velocity in the perpendicular direction. Moreover, in some of the points the transverse velocity oscillates as a function of the radiation intensity. These features of the quasiparticle spectrum manifest themselves in the conductance of graphene-based junctions in the regime of strong irradiation. For instance, we find that the conductance of a graphene p-n junction depends on the polarization as $G(\theta)\propto|\sin\theta|^{3/2}$, where $\theta$ is the angle between the polarization and the p-n interface, and oscillates as a function of the radiation intensity.