Optically detected impurity D⁰ and D^- transitions in GaAs quantum wells

We have carried out a far-infrared magnet-optical study on shallow donor states confined in GaAs quantum wells (QWs), applying a recently developed optical detection technique. We have observed, in addition to cyclotron resonance, the 1s → 2p₊ transition of neutral donors (D⁰), and singlet and triplet transitions of negative donor ions (D^-); the latter observation verifies the existence of D^- ions in well-only doped QWs under optical pumping. This is the first observation of optically detected impurity resonances in confined systems and demonstrates the power and utility of this technique for such studies.