  Terahertz field induced photocurrents in graphene were studied experimentally
and by microscopic modeling. Currents were generated by cw and pulsed laser
radiation in large area as well as small-size exfoliated graphene samples. We
review general symmetry considerations leading to photocurrents depending on
linear and circular polarized radiation and then present a number of situations
where photocurrents were detected. Starting with the photon drag effect under
oblique incidence, we proceed to the photogalvanic effect enhancement in the
reststrahlen band of SiC and edge-generated currents in graphene. Ratchet
effects were considered for in-plane magnetic fields and a structure inversion
asymmetry as well as ratchets by non-symmetric patterned top gates. Lastly, we
demonstrate that graphene can be used as a fast, broadband detector of
terahertz radiation.
