Terahertz ratchet effects in graphene with a lateral superlattice

Experimental and theoretical studies on ratchet effects in graphene with a\nlateral superlattice excited by alternating electric fields of terahertz\nfrequency range are presented. A lateral superlatice deposited on top of\nmonolayer graphene is formed either by periodically repeated metal stripes\nhaving different widths and spacings or by inter-digitated comb-like\ndual-grating-gate (DGG) structures. We show that the ratchet photocurrent\nexcited by terahertz radiation and sensitive to the radiation polarization\nstate can be efficiently controlled by the back gate driving the system through\nthe Dirac point as well as by the lateral asymmetry varied by applying unequal\nvoltages to the DGG subgratings. The ratchet photocurrent includes the Seebeck\nthermoratchet effect as well as the effects of "linear" and "circular"\nratchets, sensitive to the corresponding polarization of the driving\nelectromagnetic force. The experimental data are analyzed for the electronic\nand plasmonic ratchets taking into account the calculated potential profile and\nthe near field acting on carriers in graphene. We show that the photocurrent\ngeneration is based on a combined action of a spatially periodic in-plane\npotential and the spatially modulated light due to the near field effects of\nthe light diffraction.\n