Helicity-dependent photocurrents in graphene layers excited by   mid-infrared radiation of a CO$_2$-laser
  We report the study of the helicity driven photocurrents in graphene excited by mid-infrared light of a CO$_2$-laser. Illuminating an unbiased monolayer sheet of graphene with circularly polarized radiation generates -- under oblique incidence -- an electric current perpendicular to the plane of incidence, whose sign is reversed by switching the radiation helicity. We show that the current is caused by the interplay of the circular $ac$ Hall effect and the circular photogalvanic effect. Studying the frequency dependence of the current in graphene layers grown on the SiC substrate we observe that the current exhibits a resonance at frequencies matching the longitudinal optical phonon in SiC. 
Chongyun Jiang, V.A. Shalygin, V.Yu. Panevin, S.N. Danilov, M.M.   Glazov, R. Yakimova, S. Lara-Avila, S. Kubatkin, and S.D....
cond-mat.mes-hall