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