  We report the first terahertz study of the intra-excitonic 1s-2p transition
at high excitation densities in GaAs/AlGaAs quantum wells. A strong shift,
broadening, and ultimately the disappearance of this resonance occurs with
increasing density, after ultrafast photoexcitation at the near-infrared
exciton line. Densities of excitons and unbound electron-hole pairs are
followed quantitatively using a model of the composite terahertz dielectric
response. Comparison with near-infrared absorption changes reveals a
significantly enhanced energy shift and broadening of the intra-excitonic
resonance.
