COHERENT AND INCOHERENT TUNNELING IN ASYMMETRIC DOUBLE QUANTUM-WELLS

We discuss tunnelling processes in double quantum well structures in two limiting cases. Firstly, we report the coherent oscillations of a photogenerated electronic wavepacket between the wells. The THz oscillations are traced by an optical pump-probe as well as by four-wave-mixing spectroscopy. A comparison with theory shows that the two experiments give complementary information about the various relaxation processes. 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 tunnelling process is considerably slowed down by the scattering and relaxation processes.