Ratchet effects induced by terahertz radiation in heterostructures with   a lateral periodic potential
  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. 
P. Olbrich, E.L. Ivchenko, T. Feil, R. Ravash, S.D. Danilov, J.   Allerdings, D. Weiss, and S.D. Ganichev