Strongly anisotropic Dirac quasiparticles in irradiated graphene

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. We find that the conductance of a graphene p-n junction in the regime of strong irradiation depends on the polarization as $G\left(\theta \right)\propto {|sin\theta |}^{3/2}$, where $\theta$ is the angle between the polarization and the p-n interface, and oscillates as a function of the radiation intensity.

Figure 1

Stages of evolution of the quasiparticle wave function. Landau-Zener tunneling between states $|\uparrow \rangle$ and $|\downarrow \rangle$ occurs at times a, b, and c. Between these moments the evolution is adiabatic.

Figure 2

Floquet spectra of quasiparticles in strongly irradiated graphene.

Figure 3

The locations of the Dirac points in the Floquet spectrum.

Figure 4

Irradiated graphene junction.