Magneto-spectroscopy of two-dimensional systems: Many- and few-body effects

We present a classification of states and exact optical selection rules for charged electron-hole (e-h) complexes in magnetic fields that follow from magnetic translations and axial symmetry. A possibility to observe photoluminescence of dark X^- states in angle-resolved experiments is discussed. The effect of excess electrons on internal transitions of negatively charged excitons X^- in GaAs quantum wells is studied theoretically and experimentally (by optically detected resonance (ODR) spectroscopy). An experimentally observed blue shift with excess electron density is explained in terms of collective excitations-magnetoplasmons bound to a mobile valence band hole.