  We study terahertz radiation induced ratchet currents in low dimensional
semiconductor structures with a superimposed one-dimensional lateral periodic
potential. The periodic potential is produced by etching a grating into the
sample surface or depositing metal stripes periodically on the sample top.
Microscopically, the photocurrent generation is based on the combined action of
the lateral periodic potential, verified by transport measurements, and the
in-plane modulated pumping caused by the lateral superlattice. We show that a
substantial part of the total current is caused by the polarization-independent
Seebeck ratchet effect. In addition, polarization-dependent photocurrents
occur, which we interpret in terms of their underlying microscopical
mechanisms. As a result, the class of ratchet systems needs to be extended by
linear and circular ratchets, sensitive to linear and circular polarizations of
the driving electro-magnetic force.
