Effects of collective excitations and scatterings in quantum well intersubband transitions

Quantum well intersubband transitions display some of the most interesting many-body effects including various collective excitations, such as intersubband plasmon and Fermi-edge singularity (repellon). To describe these various excitations and to study the effects of scatterings on the intersubband lineshape, we have performed a systematic microscopic theoretical investigation of intersubband transitions. The theory leads to a set of intersubband semiconductor Bloch equations (ISBEs) at the first order of Coulomb interaction. The extension to include the second order Coulomb interaction and LO-phonon interaction leads to optical dephasing or linewidth broadening. Using this theory, we have studied systematically the interplay of collective excitations in quantum well intersubband transitions. Our results show that such interplay leads to dramatic changes in spectral features, such as absorption peak positions and lineshape, compared with a free-particle theory. We will also show that the typical usage of the dephasing rate approximation is generally invalid for intersubband transitions. There is a strong cancellation effects between the in- (off-diagonal) and out(diagonal) scattering terms at the second order and the out-scattering alone overestimates the linewidth significantly. Such a cancellation is much stronger for intersubband transitions than for interband case, because of the much smaller inhomogeneous broadening in the intersubband case. We also show that there is a cancellation of electron-electron and electron-phonon scatterings in their contributions to the linewidth. Finally we will show that our theory agrees very well with recent infrared absorption experiments.