Effect of radiation on transport in graphene

We study transport properties of graphene-based $p\text{−}n$ junctions irradiated by 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.

Figure 1

(Color online) Graphene junction in presence of external radiation. The $z$ axis is directed along the electron momentum $\mathbf{p}$ in the graphene plane and the $x$ axis parallel to the electric field $\mathbf{E}$ in the external electromagnetic wave.

Figure 2

Scattering of quasiparticles moving perpendicularly to the barrier.

Figure 3

Tunneling through the dynamical gap in the momentum space (rotating RF). The quasiparticle spectrum in presence of radiation [Eq. (3.10)] is shown by the solid lines in the plot and the spectrum in absence of radiation by the thin lines.

Figure 4

(Color online) Radiation-induced hop to the trajectory with opposite pseudospin at the resonant point. Moving initially along arc AO, at the resonant point O the quasiparticle either hops to the trajectory OD, if scattered by the radiation, or continues moving along OB, in case the scattering has not occurred. Axis $\xi$ of the coordinate system is directed along trajectory AOB at each point, axis $\zeta$ is perpendicular to the graphene plane, and $\eta$ lies in the plane.

Figure 5

(Color online) Process contributing to the current through the junction (solid lines). The particle emits a photon at the first resonant point and passes without reflection the second (2D modes). (a) In the plot “kinetic energy vs longitudinal momentum.” (b) In the spatial coordinate space (graphene plane view). The emission of the photon is accompanied by the hop at the resonant point between two classical paths, obtained in absence of radiation.

Figure 6

(Color online) Another process contributing to the current through the junction (solid lines). The particle emits a photon at the first resonant point and passes without reflection the second (2D modes). (a) In the plot kinetic energy vs longitudinal momentum. (b) In the spatial coordinate space (graphene plane view). The emission of the photon is accompanied by the hop at the resonant point between two classical paths, obtained in absence of radiation.

Figure 7

Kinetic energies of the particles incident from the left reservoir and scattered back there (2D modes). (a) The particle emits and absorbs a photon at the first and the second resonant points correspondingly. (b) The particle is not scattered by the radiation.

Figure 8

Kinetic energies of the particles incident from the left reservoir and penetrating into the right one (normal modes). (a) Usual Klein tunneling, no radiation-induced scattering. (b) Tunneling accompanied by the two-photon emission.

Figure 9

(Color online) Contribution of electrons with different energies into the photocurrent through graphene $p\text{−}n$ junction.

Figure 10

Photocurrent in a graphene $p\text{−}n$ junction as a function of the radiation intensity.

Figure 11

Radiation-induced excitation processes, which contribute to the photocurrent in the graphene $p\text{−}n$ junction.