  We report on the observation of terahertz radiation induced photogalvanic
currents in semiconductor heterostructures with one-dimensional lateral
periodic potential. The potential is produced by etching a grating into the
sample surface. The electric current response is well described by
phenomenological theory including both the circular and linear photogalvanic
effects. Experimental data demonstrate that the inversion asymmetry of the
periodic lateral pattern can be varied by means of electron beam lithography to
produce classical lateral ratchets. A novel microscopical mechanism for the
polarization-dependent photogalvanic effects has been proposed to interpret the
experimental findings. The photocurrent generation is based on the combined
action of the lateral periodic potential and the modulated in-plane pumping.
The latter modulation stems from near-field effects of the radiation
propagating through the grating.
