Hole mixing and strain effects in the conduction intersubband transitions of undoped quantum wells

We study the effects of hole dispersion and mixing in the conduction intersubband transitions and infrared dressing of undoped quantum wells (QWs). This is done considering transitions of photo-excited electrons from one conduction subband (e1) to another (e2) in the presence of Coulomb interaction with the photo-excited holes. We show that, when the dispersion of the hole subband hh1 (lh1) is mainly parabolic, these transitions occur mainly between the s-states of e1-hh1 (e1-lh1) and e2-hh1 (e2-lh1) excitons with the same principal quantum numbers (allowed transitions). When the hole subbands have nonparabolic dispersions, however, such transitions are suppressed while another type of intersubband transitions in which the initial and final exciton states have different principal quantum numbers (nonallowed transitions) are enhanced. We show the enhancement and suppression processes reach their maxima when hh1 and lh1 are about to cross over, allowing multilevel mixing of excitons to occur when the undoped QW interacts with a single intense infrared field polarized along its growth. We associate these results with spinor mixing of hh1 and lh1 and illustrate how via changing the spinor contributions in these subbands one can employ strain to manipulate the dipole moments of the intersubband transitions.