Terahertz Radiation Driven Chiral Edge Currents in Graphene

We observe photocurrents induced in single-layer graphene samples by illumination of the graphene edges with circularly polarized terahertz radiation at normal incidence. The photocurrent flows along the sample edges and forms a vortex. Its winding direction reverses by switching the light helicity from left to right handed. We demonstrate that the photocurrent stems from the sample edges, which reduce the spatial symmetry and result in an asymmetric scattering of carriers driven by the radiation electric field. The developed theory based on Boltzmann’s kinetic equation is in a good agreement with the experiment. We show that the edge photocurrents can be applied for determination of the conductivity type and the momentum scattering time of the charge carriers in the graphene edge vicinity.

Figure 1

Photocurrent in sample $\#1–4H$ as a function of the angle $\phi$ defining the light polarization. Solid line is a fit to Eq. (1) [see also Eq. (7) and discussion]. The inset shows the experimental geometry. The ellipses on top illustrate the polarization states for various $\phi$.

Figure 2

Photocurrent $J_A$ in sample $\#1–4H$ as a function of the laser spot position. The laser spot is scanned along $y$ and the current is picked up from two contact pairs at the top (red circles) or bottom (blue full circles) sample edges aligned along $x$ (see inset). Dashed lines represent the laser beam spatial distribution, which is measured by a pyroelectric camera, scaled to the current maximum. The bottom inset shows the scan for sample $\#3–6H$.

Figure 3

(a) Experimental geometry for the study of edge photocurrents. (b) Schematic illustration of the edge current generation. The electric field of circularly polarized radiation rotates clockwise or counterclockwise resulting in a circular motion of carriers, which is sketched by red and blue trajectories, respectively. Our theoretical model, see Eq. (7), shows that the circular edge current stems from carriers moving towards the edge. It is due to the second order $\mathit{E}$-field correction to the distribution function and involves the retardation of the electron motion with respect to the instantaneous electric field. Switching the radiation helicity reverses the motion direction and, consequently, the electric current. (c) and (d) photocurrent topology. Red and blue arrows show the current direction for ${\sigma }_{+}$ and ${\sigma }_{-}$ polarizations, respectively, at $f=2\mathrm{THz}$. Numbers indicate the photocurrent amplitude $J_A$ in microampers.

Figure 4

Photocurrent $J_A$ measured for different edge segments. Lines are fits to Eq. (7). The fitting parameters $\tau /{10}^{-14}\mathrm{s}$ for sample $\#1–4H$ and $\#2–4H$ are indicated by numbers. The inset shows the measured circular photocurrent $J_A(\omega \tau )$ at one of the edge segments of sample $\#1–4H$ (open circles) together with the fit after Eq. (7). Data point at $\omega \tau =4.4$ is obtained applying a pulsed ${\mathrm{CO}}_2$ laser.