OPTICALLY DETECTED FAR-INFRARED RESONANCES IN DOPED GAAS QUANTUM-WELLS

We have observed a variety of far-infrared resonances, including the t ransition from the ground state to the first excited state in neutral donors, and singlet and triplet transitions of negative donor ions (D- ), as well as electron-cyclotron resonance, in well-center-doped GaAs quantum wells, employing a recently developed optical detection techni que. The power of this technique for studying impurity states in confi ned systems is clearly revealed. Results provide evidence for the exis tence of D- centers under optical excitation in multiple-quantum-well structures doped only in the wells.