Optically detected far-infrared resonances in doped GaAs quantum wells

We have observed a variety of far-infrared resonances, including the transition from the ground state to the first excited state in neutral donors, and singlet and triplet transitions of negative donor ions (${\mathit{D}}^{\mathrm{-}}$), as well as electron-cyclotron resonance, in well-center-doped GaAs quantum wells, employing a recently developed optical detection technique. The power of this technique for studying impurity states in confined systems is clearly revealed. Results provide evidence for the existence of ${\mathit{D}}^{\mathrm{-}}$ centers under optical excitation in multiple-quantum-well structures doped only in the wells.