COHERENT AND INCOHERENT TUNNELING IN ASYMMETRIC DOUBLE QUANTUM-WELLS

We discuss tunnelling processes in double quantum well structures in t wo limiting cases. Firstly, we report the coherent oscillations of a p hotogenerated electronic wavepacket between the wells. The THz oscilla tions are traced by an optical pump-probe as well as by four-wave-mixi ng spectroscopy. A comparison with theory shows that the two experimen ts give complementary information about the various relaxation process es. Secondly, we discuss hole tunnelling in weakly coupled wells as an example of resonant tunnelling in the limit of strong scattering. The experimental and theoretical results show that the resonant tunnellin g process is considerably slowed down by the scattering and relaxation processes.