Quantum ratchet effects induced by terahertz radiation in GaN-based   two-dimensional structures
  Photogalvanic effects are observed and investigated in wurtzite (0001)-oriented GaN/AlGaN low-dimensional structures excited by terahertz radiation. The structures are shown to represent linear quantum ratchets. Experimental and theoretical analysis exhibits that the observed photocurrents are related to the lack of an inversion center in the GaN-based heterojunctions. 
W. Weber, L. E.Golub, S. N. Danilov, J. Karch, C. Reitmaier, B.   Wittmann, V. V. Bel'kov, E. L. Ivchenko, Z. D. Kvon, N. Q....