Coherent Transport

The coherent tunneling of carriers includes the spatial transfer of charges. In case of asymmetric double quantum wells (ADQWs) coherent spatial oscillation can arise which are coupled to an oscillation of a macroscopic polarization density. The latter can be made visible in ultra-fast optical experiments (probing oscillating inter-band polarization) or via the associated emission of THz radiation (due to an oscillating intra-band polarization). A related phenomenon is the coherent oscillation of carriers in momentum space called Bloch oscillations. Due to the fast carrier scattering such oscillations requiring multiple transfer through a full (mini-)Brillouin zone can only occur in semiconductor superlattices. Since these oscillations are also connected with coherent oscillations in real space similar experimental methods can be used for detection. The Wannier-Stark ladder is introduced for a more elaborate description of Bloch oscillations and an analogy is described to the cyclotron motion of charges in a magnetic field. This chapter introduces the experimental technique of THz spectroscopy.