  The nature of the correlated electron gas and its response to photo-injected
electron-hole pairs in nominally undoped and modulation-doped multiple
quantum-well (MQW) structures was studied by experiment and theory, revealing a
new type of optically-active excitation, magnetoplasmons bound to a mobile
valence hole. These excitations are blue-shifted from the corresponding
transition of the isolated charged magnetoexciton X-. The observed blue-shift
of X- is larger than that of two-electron negative donor D-, in agreement with
theoretical predictions.
