Abstract
The low temperature optical properties of semiconductor quantum wells are dominated by excitonic complexes, i.e. a few charges bound together by the mutual Coulomb interaction. Excitonic complexes have been widely studied in the past not only for their importance in the physics of semiconductors, but also because they represent model systems for the interaction between a small number of charges. This thesis studies the motion of the centre-of-mass of neutral excitons (one electron and one hole) and negatively charged excitons (two electrons and one hole) with respect to the host semiconductor. The first three chapters are introductory to the subject of the thesis. In particular the third chapter explains the details of the imaging technique used to measure the motion of excitons. The fourth chapter shows that, against common belief, at low temperature excitons are very mobile. In particular the diffusive regime breaks down for T<20 K, provided a very low disorder in the quantum well, and to explain the data it is necessary to consider the contribution of exciton-polaritons. Chapter 5 regards the free or localised nature of charged excitons, which has created quite a debate since the first obseravtion of this complex. It is shown that in high quality quantum wells, negatively charged excitons drift in the presence of an in-plane electric field, which demonstrates their existence as free negative particles. Moreover the results show the possibility to manipulate optical excitations with externally applied fields. The experiment described in chapter 6 studied the different effect of an external magnetic field on the motion of neutral and negatively charged excitons. It is found that in sample with high disorder, neutral excitons diffuse and form charged excitons locally, by capturing extra electrons present in the sample, according to a dynamical equilibrium. The magnetic field inhibits the formation of charged exciton from the excitons by polarising the extra electrons. Finally chapter 7 shows the possibility to controlling the motion also of neutral excitons, by demonstrating the possibility to generate a very high force in the quantum well by magnetic field gradients
